Add Support to Visually Identify Banned Users

[BrokenEagle]

@evazion brought up a nifty idea in his DanbooruEx forum topic, which allows for the easy visual identification of banned users, which is useful for distinguishing which new users have already been banned due to vandalism (or other reasons). With that, already banned users can be ignored on the Users Listing and focus can instead be brought upon other new users. However, Evazion's implementation is currently being done with Javascript.

It would be nice to have a CSS custom setting instead to visually identify these users. This could be done the same as how the other permissions are identified, i.e. through the link_to_user function in application_helper.rb with the following additional line:

user_class = user_class + " user-banned" if user.is_banned?