[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)

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)