Re-ordering of directory structure

Replaced the whole gamemodes directory with a modifications text file.
I originally planned to use git to track modifications, but this way is easier without forcing us to fork the garrysmod repository.

The addons directory has been changed.
All CityMayhem lua scripts go inside /addons/cmlua
NO CUSTOM CONTENT SHOULD BE INSIDE HERE!
Regarding other addon directories, most of them have a config.txt file and modifications.txt file (if needed).

If possible, other addons have not been added to this repository to keep the repository small, and since there's no need.