[GITEA] Allow user to select email for file operations in Web UI

- Add a dropdown to the web interface for changing files to select which
Email should be used for the commit. It only shows (and verifies) that a
activated mail can be used, while this isn't necessary, it's better to
have this already in place.
- Added integration testing.
- Resolves https://codeberg.org/forgejo/forgejo/issues/281

(cherry picked from commit 564e701f40)
(cherry picked from commit de8f2e03cc)
(cherry picked from commit 0182cff12e)
(cherry picked from commit 9c74254d46)
(cherry picked from commit 2f0b68f821)
(cherry picked from commit 079b995d49)

tests/integration/api_repo_languages_test.go

@@ -26,11 +26,12 @@ func TestRepoLanguages(t *testing.T) {
 
 		// Save new file to master branch
 		req = NewRequestWithValues(t, "POST", "/user2/repo1/_new/master/", map[string]string{
-			"_csrf":         doc.GetCSRF(),
-			"last_commit":   lastCommit,
-			"tree_path":     "test.go",
-			"content":       "package main",
-			"commit_choice": "direct",
+			"_csrf":          doc.GetCSRF(),
+			"last_commit":    lastCommit,
+			"tree_path":      "test.go",
+			"content":        "package main",
+			"commit_choice":  "direct",
+			"commit_mail_id": "3",
 		})
 		session.MakeRequest(t, req, http.StatusSeeOther)