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.

routers/api/v1/convert/convert.go

@@ -12,6 +12,7 @@ import (
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/structs"
 	api "code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/util"
 
@@ -84,17 +85,21 @@ func ToCommit(repo *models.Repository, c *git.Commit) *api.PayloadCommit {
 // ToVerification convert a git.Commit.Signature to an api.PayloadCommitVerification
 func ToVerification(c *git.Commit) *api.PayloadCommitVerification {
 	verif := models.ParseCommitWithSignature(c)
-	var signature, payload string
+	commitVerification := &api.PayloadCommitVerification{
+		Verified: verif.Verified,
+		Reason:   verif.Reason,
+	}
 	if c.Signature != nil {
-		signature = c.Signature.Signature
-		payload = c.Signature.Payload
+		commitVerification.Signature = c.Signature.Signature
+		commitVerification.Payload = c.Signature.Payload
 	}
-	return &api.PayloadCommitVerification{
-		Verified:  verif.Verified,
-		Reason:    verif.Reason,
-		Signature: signature,
-		Payload:   payload,
+	if verif.SigningUser != nil {
+		commitVerification.Signer = &structs.PayloadUser{
+			Name:  verif.SigningUser.Name,
+			Email: verif.SigningUser.Email,
+		}
 	}
+	return commitVerification
 }
 
 // ToPublicKey convert models.PublicKey to api.PublicKey