Fix frontend deploy and Heroku staging (#1288)

- Heroku reads the Node version from package.json. We're about to upgrade to Node 8 so this change to `engines` is only pre-emptive. It won't have any effect on the production servers.
- The production deploy scripts were missing the frontend files. This fixes that.
- This modifies the build setup to allow `BASE_URL=/`, which makes all requests relative to the page itself. That simplifies deploying the "debugging" frontend to the production servers, and makes it easy to host the frontend on Heroku.

package-lock.json

@@ -245,6 +245,21 @@
         }
       }
     },
+    "JSONSelect": {
+      "version": "0.4.0",
+      "resolved": "https://registry.npmjs.org/JSONSelect/-/JSONSelect-0.4.0.tgz",
+      "integrity": "sha1-oI7cxn6z/L6Z7WMIVTRKDPKCu40="
+    },
+    "JSONStream": {
+      "version": "1.3.1",
+      "resolved": "https://registry.npmjs.org/JSONStream/-/JSONStream-1.3.1.tgz",
+      "integrity": "sha1-cH92HgHa6eFvG8+TcDt4xwlmV5o=",
+      "dev": true,
+      "requires": {
+        "jsonparse": "1.3.1",
+        "through": "2.3.8"
+      }
+    },
     "abab": {
       "version": "1.0.4",
       "resolved": "https://registry.npmjs.org/abab/-/abab-1.0.4.tgz",
@@ -2670,8 +2685,8 @@
       "integrity": "sha512-8od6g684Fhi5Vpp4ABRv/RBsW1AY6wSHbJHEK6FGTv+8jvAAnlABniZu/FVmX9TcirkHepaEsa1QGkRvbg0CKw==",
       "dev": true,
       "requires": {
-        "is-text-path": "1.0.1",
         "JSONStream": "1.3.1",
+        "is-text-path": "1.0.1",
         "lodash": "4.17.4",
         "meow": "3.7.0",
         "split2": "2.2.0",
@@ -5237,15 +5252,6 @@
             }
           }
         },
-        "string_decoder": {
-          "version": "1.0.1",
-          "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.0.1.tgz",
-          "integrity": "sha1-YuIA8DmVWmgQ2N8KM//A8BNmLZg=",
-          "dev": true,
-          "requires": {
-            "safe-buffer": "5.0.1"
-          }
-        },
         "string-width": {
           "version": "1.0.2",
           "resolved": "https://registry.npmjs.org/string-width/-/string-width-1.0.2.tgz",
@@ -5257,6 +5263,15 @@
             "strip-ansi": "3.0.1"
           }
         },
+        "string_decoder": {
+          "version": "1.0.1",
+          "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.0.1.tgz",
+          "integrity": "sha1-YuIA8DmVWmgQ2N8KM//A8BNmLZg=",
+          "dev": true,
+          "requires": {
+            "safe-buffer": "5.0.1"
+          }
+        },
         "stringstream": {
           "version": "0.0.5",
           "resolved": "https://registry.npmjs.org/stringstream/-/stringstream-0.0.5.tgz",
@@ -6548,12 +6563,12 @@
       "resolved": "https://registry.npmjs.org/jison/-/jison-0.4.13.tgz",
       "integrity": "sha1-kEFwfWIkE2f1iDRTK58ZwsNvrHg=",
       "requires": {
+        "JSONSelect": "0.4.0",
         "cjson": "0.2.1",
         "ebnf-parser": "0.1.10",
         "escodegen": "0.0.21",
         "esprima": "1.0.4",
         "jison-lex": "0.2.1",
-        "JSONSelect": "0.4.0",
         "lex-parser": "0.1.4",
         "nomnom": "1.5.2"
       },
@@ -6805,21 +6820,6 @@
       "integrity": "sha1-T9kss04OnbPInIYi7PUfm5eMbLk=",
       "dev": true
     },
-    "JSONSelect": {
-      "version": "0.4.0",
-      "resolved": "https://registry.npmjs.org/JSONSelect/-/JSONSelect-0.4.0.tgz",
-      "integrity": "sha1-oI7cxn6z/L6Z7WMIVTRKDPKCu40="
-    },
-    "JSONStream": {
-      "version": "1.3.1",
-      "resolved": "https://registry.npmjs.org/JSONStream/-/JSONStream-1.3.1.tgz",
-      "integrity": "sha1-cH92HgHa6eFvG8+TcDt4xwlmV5o=",
-      "dev": true,
-      "requires": {
-        "jsonparse": "1.3.1",
-        "through": "2.3.8"
-      }
-    },
     "jsprim": {
       "version": "1.4.1",
       "resolved": "https://registry.npmjs.org/jsprim/-/jsprim-1.4.1.tgz",
@@ -11852,11 +11852,6 @@
       "resolved": "https://registry.npmjs.org/strict-uri-encode/-/strict-uri-encode-1.1.0.tgz",
       "integrity": "sha1-J5siXfHVgrH1TmWt3UNS4Y+qBxM="
     },
-    "string_decoder": {
-      "version": "0.10.31",
-      "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-0.10.31.tgz",
-      "integrity": "sha1-YuIDvEF2bGwoyfyEMB2rHFMQ+pQ="
-    },
     "string-hash": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/string-hash/-/string-hash-1.1.1.tgz",
@@ -11899,6 +11894,11 @@
         }
       }
     },
+    "string_decoder": {
+      "version": "0.10.31",
+      "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-0.10.31.tgz",
+      "integrity": "sha1-YuIDvEF2bGwoyfyEMB2rHFMQ+pQ="
+    },
     "stringstream": {
       "version": "0.0.5",
       "resolved": "https://registry.npmjs.org/stringstream/-/stringstream-0.0.5.tgz",
@@ -12686,6 +12686,12 @@
         }
       }
     },
+    "url-path": {
+      "version": "0.1.1",
+      "resolved": "https://registry.npmjs.org/url-path/-/url-path-0.1.1.tgz",
+      "integrity": "sha512-DQByXz6WBWMHVlRRuSgJrW/UYPFAyGnBTpeSB4LWN5iGe7sVuUEQsh3j9Wh34T+6l4Ftyy9ezbme1nY2jvRUEg==",
+      "dev": true
+    },
     "util": {
       "version": "0.10.3",
       "resolved": "https://registry.npmjs.org/util/-/util-0.10.3.tgz",