[Drone] Add Drone build badge (#3240)

* Add drone build badge based on travis

* Fix wrong mocked endpoint for done builder

* Refactor service tester using helper method

* Add missing failure status to red statuses

* Remove extraneous invalid svg test from drone

* Test on failure red status in build status spec

* refactor(drone): use json service instead of svg

* refactor(drone): remove status text and extraneous build path in test

* refactor(drone): allow defining self-hosted drone instances

* fix(drone): use proper urls in drone examples

* fix(drone): add drone token authorization for self-hosted instances

* refactor(drone): call render build status badge directly instead of render

* refactor(drone): use server query parameter for self-hosted instances

* fix(drone): separate url and query params in example

* fix(drone): use actual build status message in examples

* fix(drone): add missing message for status code 401

Co-Authored-By: byCedric <me@bycedric.com>

* refactor(drone): remove color from drone tests

* refactor(drone): remove extraneous comments from drone tests

* refactor(drone): remove unused static preview method

* refactor(drone): remove unused static render method

* refactor(drone): reuse render build status badge helper in static previews

* fix(drone): test inaccessible repos on new message

doc/server-secrets.md

@@ -43,6 +43,13 @@ An Azure DevOps Token (PAT) is required for accessing [private Azure DevOps proj
 The bintray API [requires authentication](https://bintray.com/docs/api/#_authentication)
 Create an account and obtain a token from the user profile page.
 
+## Drone
+
+- `DRONE_TOKEN` (yml: `drone_token`)
+
+The self-hosted Drone API [requires authentication](https://0-8-0.docs.drone.io/api-authentication/)
+Login to your Drone instance and obtain a token from the user profile page.
+
 ## GitHub
 
 - `GH_TOKEN` (yml: `gh_token`)