Inject secrets into the services (#3652)

This is a reworking of #3410 based on some feedback @calebcartwright left on that PR.

The goals of injecting the secrets are threefold:

1. Simplify testing
2. Be consistent with all of the other config (which is injected)
3. Encapsulate the sensitive auth-related code in one place so it can be studied and tested thoroughly

- Rather than add more code to BaseService to handle authorization logic, it delegates that to an AuthHelper class.
- When the server starts, it fetches the credentials from `config` and injects them into `BaseService.register()` which passes them to `invoke()`.
- In `invoke()` the service's auth configuration is checked (`static get auth()`, much like `static get route()`).
- If the auth config is present, an AuthHelper instance is created and attached to the new instance.
- Then within the service, the password, basic auth config, or bearer authentication can be accessed via e.g. `this.authHelper.basicAuth` and passed to `this._requestJson()` and friends.
- Everything is being done very explicitly, so it should be very clear where and how the configured secrets are being used.
- Testing different configurations of services can now be done by injecting the config into `invoke()` in `.spec` files instead of mocking global state in the service tests as was done before. See the new Jira spec files for a good example of this.

Ref #3393

dangerfile.js

@@ -112,10 +112,13 @@ if (allFiles.length > 100) {
 
     // eslint-disable-next-line promise/prefer-await-to-then
     danger.git.diffForFile(file).then(({ diff }) => {
-      if (diff.includes('serverSecrets') && !secretsDocs.modified) {
+      if (
+        (diff.includes('authHelper') || diff.includes('serverSecrets')) &&
+        !secretsDocs.modified
+      ) {
         warn(
           [
-            `:books: Remember to ensure any changes to \`serverSecrets\` `,
+            `:books: Remember to ensure any changes to \`config.private\` `,
             `in \`${file}\` are reflected in the [server secrets documentation]`,
             '(https://github.com/badges/shields/blob/master/doc/server-secrets.md)',
           ].join('')