[permissions] move permissions to database, add GUI permissions editor

[discomrade]

Closes #1139

This does not handle existing user-defined classes (e.g. data/config/user-classes.conf.php) so I suggest the next release mention that users should remove from any custom user classes before upgrading.

This pull also implies changes to the wiki: https://github.com/shish/shimmie2/wiki/Advanced-Config