[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

[shish]

So this PR has been haunting me for months because I've wanted to get it merged, but something has felt off, but I wasn't able to say what -- turns out my worries were nearly all in my own code that this builds on top of 😆 So now that I've gone and redone how permissions are defined (putting them in per-extension files, rather than one central list), I'm now working on merging this on top of that /o/

One noteworthy change being that I'm thinking of making the permissions table more like permissions(user_class, permission_id, allowed) rather than having a separate column-per-permission -- the new foundations allow for adding and removing permissions when extensions are enabled or disabled, and doing a database schema change in each case sounds scary >.>;;