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.

options/locale/locale_en-US.ini

@@ -1974,12 +1974,15 @@ mark_as_unread = Mark as unread
 mark_all_as_read = Mark all as read
 
 [gpg]
+default_key=Signed with default key
 error.extract_sign = Failed to extract signature
 error.generate_hash = Failed to generate hash of commit
 error.no_committer_account = No account linked to committer's email address
 error.no_gpg_keys_found = "No known key found for this signature in database"
 error.not_signed_commit = "Not a signed commit"
 error.failed_retrieval_gpg_keys = "Failed to retrieve any key attached to the committer's account"
+error.probable_bad_signature = "WARNING! Although there is a key with this ID in the database it does not verify this commit! This commit is SUSPICIOUS."
+error.probable_bad_default_signature = "WARNING! Although the default key has this ID it does not verify this commit! This commit is SUSPICIOUS."
 
 [units]
 error.no_unit_allowed_repo = You are not allowed to access any section of this repository.