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/repofiles/temp_repo.go

@@ -261,7 +261,6 @@ func (t *TemporaryUploadRepository) CommitTree(author, committer *models.User, t
 		return "", fmt.Errorf("Unable to get git version: %v", err)
 	}
 
-	// FIXME: Should we add SSH_ORIGINAL_COMMAND to this
 	// Because this may call hooks we should pass in the environment
 	env := append(os.Environ(),
 		"GIT_AUTHOR_NAME="+authorSig.Name,
@@ -271,13 +270,21 @@ func (t *TemporaryUploadRepository) CommitTree(author, committer *models.User, t
 		"GIT_COMMITTER_EMAIL="+committerSig.Email,
 		"GIT_COMMITTER_DATE="+commitTimeStr,
 	)
+
 	messageBytes := new(bytes.Buffer)
 	_, _ = messageBytes.WriteString(message)
 	_, _ = messageBytes.WriteString("\n")
 
 	args := []string{"commit-tree", treeHash, "-p", "HEAD"}
-	if version.Compare(binVersion, "2.0.0", ">=") {
-		args = append(args, "--no-gpg-sign")
+
+	// Determine if we should sign
+	if version.Compare(binVersion, "1.7.9", ">=") {
+		sign, keyID := t.repo.SignCRUDAction(author, t.basePath, "HEAD")
+		if sign {
+			args = append(args, "-S"+keyID)
+		} else if version.Compare(binVersion, "2.0.0", ">=") {
+			args = append(args, "--no-gpg-sign")
+		}
 	}
 
 	commitHash, stderr, err := process.GetManager().ExecDirEnvStdIn(5*time.Minute,