Templates Legacy: Single Groups Admin Screens

Group admin template managing the settings & create steps sub screens has become a file which is hard for developers to navigate through to edit.

Commit re-factors the admin.php file to run as a primary include file checking for the current action variable to load the required template part.

Commit breaks out the sections representing settings /creation steps from the admin template to their own template include files located in a new directory 'single/admin/'.

Smaller split template files are now easier to manage & read for editing and overloading.

Commit re-locates admin files hidden form input to buddypress-functions.php hooking to the existing templates do_action function.

Thanks to Offereins for the original ticket - this commit follows closely the approach used in bp-nouveau templates.

Props offereins, dcavins, r-a-y

Fixes #7079