Allow Token API calls be authorized using the reverse-proxy header (#15119)

Allow Token API calls be authorized using the reverse-proxy header (#15119)

* API calls authorized with HTTP header

This mod allows API calls to be authorized with HTTP header
when ENABLE_REVERSE_PROXY_AUTHENTICATION is enabled. Without
it user authenticated by reverse proxy is able to access
gitea UI but not API which is inconsistent.

Author-Change-Id: IB#1107572

* Fixed API calls authorized with HTTP header

Only reqBasicAuth is modified to allow reverse proxy
auth as alternative and reqToken is left untouched.

Fixes: dc952c063206d11504085ddea966f121e796a04c
Author-Change-Id: IB#1107572

* Reverse proxy API auth separated in docs

Related: https://github.com/go-gitea/gitea/pull/15119#discussion_r621322127
Author-Change-Id: IB#1107572

* Reverse proxy API auth separated in docs

Related: https://github.com/go-gitea/gitea/pull/15119#discussion_r621322127
Author-Change-Id: IB#1107572

* Reverse proxy API auth separated

Related: https://github.com/go-gitea/gitea/pull/15119#discussion_r621322127
Author-Change-Id: IB#1107572

* ReverseProxyAuth removed from swagger

ReverseProxyAuth removed from swagger as in upstream's suggestion.

Related: https://github.com/go-gitea/gitea/pull/15119#pullrequestreview-692180940
Author-Change-Id: IB#1107572

* ReverseProxyAuth API authorization fixed

Related: https://github.com/go-gitea/gitea/pull/15119#issuecomment-868465099
Author-Change-Id: IB#1107572

* ReverseProxyAuth API authorization fixed

Related: https://github.com/go-gitea/gitea/pull/15119#issuecomment-868465099
Author-Change-Id: IB#1107572