Refactor UsersController#create

* Simplify controller action
* Extract service classes
added international amazon domains to matcher
changed . to \.
attempt to fix amazon item descriptions
better revision history
:scissors: scoped for Rails 4 compat
remove unused diff_engine class
Merge pull request #1729 from S-YOU/master

amazon oneboxer
Corrected the mention string in the user stream. (props to @sunyatasattva)

Previously the string would show *You mentioned {{Your Name}}*. Now it
correctly shows *You mentioned {{Other user name}}*
FIX: allow new user to post emojis
that fix should have been in the emoji plugin
FIX: images should also be inlined in the cooked view
FIX: absolute canonical link on topic pages
add image dimensions & size in lightbox
Experimental: Disqus XML importer
Merge pull request #1731 from chancancode/no_more_scoped_in_rails_4_1

:scissors: scoped for Rails 4 compat
Show more than one topic per category on the wide categories page. Disable support for the 2-column categories page. The category_featured_topics site setting controls how many topics to show.
FIX: layout problems on categories page
FIX: broken font-awesome icons on user activity pages
FIX: font-awesome issue on site settings buttons