add [twitch] user status badge (#3683)

* add /twitch/status/:user badge

* update comments

* use a proper schema for the Twitch API calls

* use a token to make Twitch api calls

* fix handling of rate-limit error and bad token error

* [twitch] get a token as soon as creating a Twitch service

* [twitch] start both requests to users and stream before awaiting

* [twitch] set a timeout to replace the token before it expires

* [twitch] use authHelper

* [twitch] skip tests when no credentials

* [twitch] add one more status test

* twitch: do not check whether a user exists

doc/server-secrets.md

@@ -141,6 +141,14 @@ Create an account, sign in and obtain a uuid and token from your
 to give your self-hosted Shields installation access to a
 private SonarQube instance or private project on a public instance.
 
+## Twitch
+
+- `TWITCH_CLIENT_ID` (yml: `twitch_client_id`)
+- `TWITCH_CLIENT_SECRET` (yml: `twitch_client_secret`)
+
+Register an application in the [Twitch developer console](https://dev.twitch.tv/console)
+in order to obtain a client id and a client secret for making Twitch API calls.
+
 ## Wheelmap
 
 - `WHEELMAP_TOKEN` (yml: `wheelmap_token`)