More user-friendly way to select users, groups, and forums in ACP

[imkingdavid]

When you are adding/editing a prefix, you can specify who can use it and where it can be used. Right now it's not userfriendly because you have to find the ID of the user/forum/group and enter it in a certain format (comma-separated).

I plan to add multi-select from groups and forums, similar to in other parts of the ACP/forum, and an auto-complete textbox to specify usernames that show up as "tags" in the text box (sort of like StackOverflow does with tags on its posts).

For the latter I'll need to investigate how the current autocompletion is implemented so I can piggyback off of that.