Refactor rename user and rename organization (#24052)

This PR is a refactor at the beginning. And now it did 4 things.
- [x] Move renaming organizaiton and user logics into services layer and
merged as one function
- [x] Support rename a user capitalization only. For example, rename the
user from `Lunny` to `lunny`. We just need to change one table `user`
and others should not be touched.
- [x] Before this PR, some renaming were missed like `agit`
- [x] Fix bug the API reutrned from `http.StatusNoContent` to `http.StatusOK`

options/locale/locale_en-US.ini

@@ -520,6 +520,7 @@ lang_select_error = Select a language from the list.
 
 username_been_taken = The username is already taken.
 username_change_not_local_user = Non-local users are not allowed to change their username.
+username_has_not_been_changed = Username has not been changed
 repo_name_been_taken = The repository name is already used.
 repository_force_private = Force Private is enabled: private repositories cannot be made public.
 repository_files_already_exist = Files already exist for this repository. Contact the system administrator.