Fixes #47 Forumpermissions module not working

#47 
I've added a `$groups` array to the different boards which contains all default groups (old_id => mybb_id) and added a function to the main boards class which resolves a gid by first checking the new array, then asking the cache about custom groups (like it was done before). I've also modified the `get_group_id` functions to use that function instead of a big switch (#101).

The forumpermissions module(s) were only checking custom groups, I've edited them to use the new function to also resolve the default ones. In addition I've fixed a bug within the phpBB module which ended in countless useless redirects.

Probably fixed a few more things I've forgotten.
@mybb/sqa @Sama34 @JovanJ Please test this ASAP as it will easily get merge conflicts.