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.

models/wiki.go

@@ -205,6 +205,13 @@ func (repo *Repository) updateWikiPage(doer *User, oldWikiName, newWikiName, con
 	commitTreeOpts := git.CommitTreeOpts{
 		Message: message,
 	}
+
+	sign, signingKey := repo.SignWikiCommit(doer)
+	if sign {
+		commitTreeOpts.KeyID = signingKey
+	} else {
+		commitTreeOpts.NoGPGSign = true
+	}
 	if hasMasterBranch {
 		commitTreeOpts.Parents = []string{"HEAD"}
 	}
@@ -307,11 +314,19 @@ func (repo *Repository) DeleteWikiPage(doer *User, wikiName string) (err error)
 		return err
 	}
 	message := "Delete page '" + wikiName + "'"
-
-	commitHash, err := gitRepo.CommitTree(doer.NewGitSig(), tree, git.CommitTreeOpts{
+	commitTreeOpts := git.CommitTreeOpts{
 		Message: message,
 		Parents: []string{"HEAD"},
-	})
+	}
+
+	sign, signingKey := repo.SignWikiCommit(doer)
+	if sign {
+		commitTreeOpts.KeyID = signingKey
+	} else {
+		commitTreeOpts.NoGPGSign = true
+	}
+
+	commitHash, err := gitRepo.CommitTree(doer.NewGitSig(), tree, commitTreeOpts)
 	if err != nil {
 		return err
 	}