Implement [YouTube] badge (#5132)

* Implement [YouTube] badge

* Update production-hosting.md with account owner

* Add votes badge variant

* Add links to tests

* Switch to social badge style

doc/production-hosting.md

@@ -24,6 +24,7 @@
 | Cloudflare                    | Admin access                    | @espadrine, @paulmelnikow                                                                  |
 | GitHub                        | OAuth app                       | @espadrine ([could be transferred to the badges org][oauth transfer])                      |
 | Twitch                        | OAuth app                       | @PyvesB                                                                                    |
+| YouTube                       | Account owner                   | @PyvesB                                                                                    |
 | OpenStreetMap (for Wheelmap)  | Account owner                   | @paulmelnikow                                                                              |
 | DNS                           | Account owner                   | @olivierlacan                                                                              |
 | DNS                           | Read-only account access        | @espadrine, @paulmelnikow, @chris48s                                                       |

doc/server-secrets.md

@@ -228,6 +228,16 @@ displayed on your profile page.
 
 [wheelmap token]: http://classic.wheelmap.org/en/users/sign_in
 
+### YouTube
+
+- `YOUTUBE_API_KEY` (yml: `private.youtube_api_key`)
+
+The YouTube API requires authentication. To obtain an API key,
+log in to a Google account, go to the [credentials page][youtube credentials],
+and create an API key for the YouTube Data API v3.
+
+[youtube credentials]: https://console.developers.google.com/apis/credentials
+
 ## Error reporting
 
 - `SENTRY_DSN` (yml: `private.sentry_dsn`)