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
FEATURE: topic support in disposable invites
FIX: Don't link to web sites for TL0
Merge pull request #2542 from techAPJ/patch-1

FEATURE: topic support in disposable invites
FIX: You shouldn't be able to add a parent to `Uncategorized`
FIX: Instead of silently failing, assign alternative username when provided username is invalid in disposable invite
Merge pull request #2545 from techAPJ/patch-1

FIX: Instead of silently failing, assign alternative username when provi...
FIX: Cooking issue with paragraphs in a list
FIX: Allow users to unsubscribe to digests while not logged in if
`login_required` is set to true.
excludes self from quoted
FIX: escape input of forgot password form before rendering it back to you
Improve the unsubscribe to digest experience. Give a link in case it
fails, provide a different message if you are logged in as a different
user, increase expiry to 2 months from 1 week.
FIX: handle enter key with a better way on site setting string inputs
fitting images in posts without violating max width/height