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/api.go

@@ -507,6 +507,7 @@ func RegisterRoutes(m *macaron.Macaron) {
 			m.Get("/swagger", misc.Swagger)
 		}
 		m.Get("/version", misc.Version)
+		m.Get("/signing-key.gpg", misc.SigningKey)
 		m.Post("/markdown", bind(api.MarkdownOption{}), misc.Markdown)
 		m.Post("/markdown/raw", misc.MarkdownRaw)
 
@@ -771,6 +772,7 @@ func RegisterRoutes(m *macaron.Macaron) {
 						m.Delete("", bind(api.DeleteFileOptions{}), repo.DeleteFile)
 					}, reqRepoWriter(models.UnitTypeCode), reqToken())
 				}, reqRepoReader(models.UnitTypeCode))
+				m.Get("/signing-key.gpg", misc.SigningKey)
 				m.Group("/topics", func() {
 					m.Combo("").Get(repo.ListTopics).
 						Put(reqToken(), reqAdmin(), bind(api.RepoTopicOptions{}), repo.UpdateTopics)