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.

integrations/api_helper_for_declarative_test.go

@@ -231,3 +231,38 @@ func doAPIMergePullRequest(ctx APITestContext, owner, repo string, index int64)
 		ctx.Session.MakeRequest(t, req, 200)
 	}
 }
+
+func doAPIGetBranch(ctx APITestContext, branch string, callback ...func(*testing.T, api.Branch)) func(*testing.T) {
+	return func(t *testing.T) {
+		req := NewRequestf(t, "GET", "/api/v1/repos/%s/%s/branches/%s?token=%s", ctx.Username, ctx.Reponame, branch, ctx.Token)
+		if ctx.ExpectedCode != 0 {
+			ctx.Session.MakeRequest(t, req, ctx.ExpectedCode)
+			return
+		}
+		resp := ctx.Session.MakeRequest(t, req, http.StatusOK)
+
+		var branch api.Branch
+		DecodeJSON(t, resp, &branch)
+		if len(callback) > 0 {
+			callback[0](t, branch)
+		}
+	}
+}
+
+func doAPICreateFile(ctx APITestContext, treepath string, options *api.CreateFileOptions, callback ...func(*testing.T, api.FileResponse)) func(*testing.T) {
+	return func(t *testing.T) {
+		url := fmt.Sprintf("/api/v1/repos/%s/%s/contents/%s?token=%s", ctx.Username, ctx.Reponame, treepath, ctx.Token)
+		req := NewRequestWithJSON(t, "POST", url, &options)
+		if ctx.ExpectedCode != 0 {
+			ctx.Session.MakeRequest(t, req, ctx.ExpectedCode)
+			return
+		}
+		resp := ctx.Session.MakeRequest(t, req, http.StatusCreated)
+
+		var contents api.FileResponse
+		DecodeJSON(t, resp, &contents)
+		if len(callback) > 0 {
+			callback[0](t, contents)
+		}
+	}
+}