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.

modules/structs/hook.go

@@ -91,10 +91,11 @@ type PayloadCommit struct {
 
 // PayloadCommitVerification represents the GPG verification of a commit
 type PayloadCommitVerification struct {
-	Verified  bool   `json:"verified"`
-	Reason    string `json:"reason"`
-	Signature string `json:"signature"`
-	Payload   string `json:"payload"`
+	Verified  bool         `json:"verified"`
+	Reason    string       `json:"reason"`
+	Signature string       `json:"signature"`
+	Signer    *PayloadUser `json:"signer"`
+	Payload   string       `json:"payload"`
 }
 
 var (