Improved Group Member Management on User Administration

Allows for a quick and easy group membership management on the
user-administration page. Uses the select2 UI component to
autosuggest other groups, remove existing ones and lock in automatic
groups.
FIX: NoMethodError in on extension.upcase when upload's original filename has no extension.
renamed
use computed
move available groups loader into router
move ajax into models
make jshint happy: missing semicolon
Emails are case insensitive
Remove hub username integration
ES6: User activity views
FEATURE: show the user's flagged/deleted posts
we can't resize images on desktop, very dangerous
FIX: We should use `category_id` instead of `category_name` to perform
operations, now that the subcategory names are not unique.
FEATURE: show a reason why a category can't be deleted
FEATURE: replace 'deleted by' with the trashcan glyph
FIX: Resolver should try with slashes instead of periods too
FIX: If the invite with topic id already exists, redirect user to home page instead
FIX: don't send repeated notifications that there are pending users who need to be approved to moderators who haven't read the previous notification
FIX: Don't scroll to top if there's an anchor present.
Merge pull request #2549 from techAPJ/patch-3

FIX: If the invite with topic id already exists, redirect user to home p...