Add support for incoming emails (#22056)

closes #13585
fixes #9067
fixes #2386
ref #6226
ref #6219
fixes #745

This PR adds support to process incoming emails to perform actions.
Currently I added handling of replies and unsubscribing from
issues/pulls. In contrast to #13585 the IMAP IDLE command is used
instead of polling which results (in my opinion 😉) in cleaner code.

Procedure:
- When sending an issue/pull reply email, a token is generated which is
present in the Reply-To and References header.
- IMAP IDLE waits until a new email arrives
- The token tells which action should be performed

A possible signature and/or reply gets stripped from the content.

I added a new service to the drone pipeline to test the receiving of
incoming mails. If we keep this in, we may test our outgoing emails too
in future.

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

docs/content/doc/features/comparison.en-us.md

@@ -106,7 +106,7 @@ _Symbols used in table:_
 | Issue search                  | ✓                                                   | ✘    | ✓         | ✓         | ✓         | ✓         | ✘            |
 | Global issue search           | [/](https://github.com/go-gitea/gitea/issues/2434)  | ✘    | ✓         | ✓         | ✓         | ✓         | ✘            |
 | Issue dependency              | ✓                                                   | ✘    | ✘         | ✘         | ✘         | ✘         | ✘            |
-| Create issue via email        | [✘](https://github.com/go-gitea/gitea/issues/6226)  | ✘    | ✘         | ✘         | ✓         | ✓         | ✘            |
+| Create issue via email        | [✘](https://github.com/go-gitea/gitea/issues/6226)  | ✘    | ✘         | ✓         | ✓         | ✓         | ✘            |
 | Service Desk                  | [✘](https://github.com/go-gitea/gitea/issues/6219)  | ✘    | ✘         | ✓         | ✓         | ✘         | ✘            |
 
 ## Pull/Merge requests