Sign merges, CRUD, Wiki and Repository initialisation with gpg key (#7631)

This PR fixes #7598 by providing a configurable way of signing commits across the Gitea instance. Per repository configurability and import/generation of trusted secure keys is not provided by this PR - from a security PoV that's probably impossible to do properly. Similarly web-signing, that is asking the user to sign something, is not implemented - this could be done at a later stage however.

## Features
- [x] If commit.gpgsign is set in .gitconfig sign commits and files created through repofiles. (merges should already have been signed.)
- [x] Verify commits signed with the default gpg as valid
- [x] Signer, Committer and Author can all be different
    - [x] Allow signer to be arbitrarily different - We still require the key to have an activated email on Gitea. A more complete implementation would be to use a keyserver and mark external-or-unactivated with an "unknown" trust level icon.
- [x] Add a signing-key.gpg endpoint to get the default gpg pub key if available
    - Rather than add a fake web-flow user I've added this as an endpoint on /api/v1/signing-key.gpg
    - [x] Try to match the default key with a user on gitea - this is done at verification time
- [x] Make things configurable?
    - app.ini configuration done
    - [x] when checking commits are signed need to check if they're actually verifiable too
- [x] Add documentation

I have decided that adjusting the docker to create a default gpg key is not the correct thing to do and therefore have not implemented this.

custom/conf/app.ini.sample

@@ -74,6 +74,37 @@ WORK_IN_PROGRESS_PREFIXES=WIP:,[WIP]
 ; List of reasons why a Pull Request or Issue can be locked
 LOCK_REASONS=Too heated,Off-topic,Resolved,Spam
 
+[repository.signing]
+; GPG key to use to sign commits, Defaults to the default - that is the value of git config --get user.signingkey
+; run in the context of the RUN_USER
+; Switch to none to stop signing completely
+SIGNING_KEY = default
+; If a SIGNING_KEY ID is provided and is not set to default, use the provided Name and Email address as the signer.
+; These should match a publicized name and email address for the key. (When SIGNING_KEY is default these are set to
+; the results of git config --get user.name and git config --get user.email respectively and can only be overrided
+; by setting the SIGNING_KEY ID to the correct ID.)
+SIGNING_NAME =
+SIGNING_EMAIL =
+; Determines when gitea should sign the initial commit when creating a repository
+; Either:
+; - never
+; - pubkey: only sign if the user has a pubkey
+; - twofa: only sign if the user has logged in with twofa
+; - always
+; options other than none and always can be combined as comma separated list
+INITIAL_COMMIT = always
+; Determines when to sign for CRUD actions
+; - as above
+; - parentsigned: requires that the parent commit is signed.
+CRUD_ACTIONS = pubkey, twofa, parentsigned
+; Determines when to sign Wiki commits
+; - as above
+WIKI = never
+; Determines when to sign on merges
+; - basesigned: require that the parent of commit on the base repo is signed.
+; - commitssigned: require that all the commits in the head branch are signed.
+MERGES = pubkey, twofa, basesigned, commitssigned
+
 [cors]
 ; More information about CORS can be found here: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#The_HTTP_response_headers
 ; enable cors headers (disabled by default)