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: Combobox wasn't matching selected value properly with value of 0
FEATURE: add site setting allow_new_registrations which can be used to block all new account registrations
FIX: Don't put iframes in emails where they are sanitized out. Replace
them with links.
make Send Test Email easier to see
FEATURE: add site settings rate_limit_new_user_create_topic and rate_limit_new_user_create_post to have different rate limits for topic and post creation for new users
improve test email copy help
FIX: cache avatars in NGINX
BUGFIX: exception validating 'data-' attributes
FIX: NoMethodError in on extension.upcase when upload's original filename has no extension.
Annotate
FIX: unique index on topic_id, post_number
FEATURE: Reader badge

Read a every post in a topic that if 50 posts or longer
FIX: First Quote badge bust
Feature: track quoted posts
Merge pull request #2540 from codetaylor/master

BUGFIX: exception validating 'data-' attributes
renamed
use computed
move available groups loader into router
SECURITY: fix XSS in link's href
move ajax into models