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/git/repo_tree.go

@@ -56,10 +56,11 @@ func (repo *Repository) GetTree(idStr string) (*Tree, error) {
 
 // CommitTreeOpts represents the possible options to CommitTree
 type CommitTreeOpts struct {
-	Parents   []string
-	Message   string
-	KeyID     string
-	NoGPGSign bool
+	Parents    []string
+	Message    string
+	KeyID      string
+	NoGPGSign  bool
+	AlwaysSign bool
 }
 
 // CommitTree creates a commit from a given tree id for the user with provided message
@@ -90,7 +91,7 @@ func (repo *Repository) CommitTree(sig *Signature, tree *Tree, opts CommitTreeOp
 	_, _ = messageBytes.WriteString(opts.Message)
 	_, _ = messageBytes.WriteString("\n")
 
-	if opts.KeyID != "" {
+	if version.Compare(binVersion, "1.7.9", ">=") && (opts.KeyID != "" || opts.AlwaysSign) {
 		cmd.AddArguments(fmt.Sprintf("-S%s", opts.KeyID))
 	}