Add new AdminTheme module type, making AdminThemes now usable and installable as modules. This also includes the ability to have multiple admin themes installed. When multiple are installed, it automatically adds a field where the user can select their theme from their profile settings.
Remove admin theme from /site/templates-admin/, since it is now converted to be a module (AdminThemeDefault).
Update system updater so that it automatically installs new admin theme module. Plus fix a bug in main Modules class that was causing it to potentially init() some modules twice.