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/setting/repository.go

@@ -65,6 +65,16 @@ var (
 		Issue struct {
 			LockReasons []string
 		} `ini:"repository.issue"`
+
+		Signing struct {
+			SigningKey    string
+			SigningName   string
+			SigningEmail  string
+			InitialCommit []string
+			CRUDActions   []string `ini:"CRUD_ACTIONS"`
+			Merges        []string
+			Wiki          []string
+		} `ini:"repository.signing"`
 	}{
 		AnsiCharset:                             "",
 		ForcePrivate:                            false,
@@ -122,6 +132,25 @@ var (
 		}{
 			LockReasons: strings.Split("Too heated,Off-topic,Spam,Resolved", ","),
 		},
+
+		// Signing settings
+		Signing: struct {
+			SigningKey    string
+			SigningName   string
+			SigningEmail  string
+			InitialCommit []string
+			CRUDActions   []string `ini:"CRUD_ACTIONS"`
+			Merges        []string
+			Wiki          []string
+		}{
+			SigningKey:    "default",
+			SigningName:   "",
+			SigningEmail:  "",
+			InitialCommit: []string{"always"},
+			CRUDActions:   []string{"pubkey", "twofa", "parentsigned"},
+			Merges:        []string{"pubkey", "twofa", "basesigned", "commitssigned"},
+			Wiki:          []string{"never"},
+		},
 	}
 	RepoRootPath string
 	ScriptType   = "bash"