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.
renamed
use computed
move available groups loader into router
move ajax into models
make jshint happy: missing semicolon
FIX: refresh invite list after sending invite manually
omit needless words
Feature: allow mods to cut pinned topic excerpts
Merge pull request #2553 from techAPJ/patch-1

FIX: refresh invite list after sending invite manually
Merge pull request #2537 from ligthyear/group-member-management-on-user

Improved Group Member Management on User Administration
Use the header background color for a site in the digest email.
Remove email_in_address and email_in_category site settings

The functionality is entirely covered by per-category email in addresses, and the category being a number was confusing people.
Remove use of email_in_address and email_in_category
Stop unnecessarily suppressing settings for uncategorized

Fix migration for emailing into uncategorized
Fix message, and add test, for EmailUnparseableError
Include the email subject line in rejection messages

This change has a tradeoff.
It increases our backscatter vulnerability - the subject could have spammy content - but it's extremely valuable to the user to know exactly which message was rejected.
If you sent two at the same time, and only one was rejected, you would have no way of knowing which worked and which to resend without going to the website (which is what email-in is trying to avoid, kinda).
Merge pull request #2530 from riking/email-2

Remove email_in_address and email_in_category site settings
FEATURE: Hide deleted posts by default for staff
Merge pull request #2546 from riking/hide_deleted

Hide deleted posts by default for staff