PNG Files optimization
Merge pull request #3 from udacity/development

Merge latest changes from upstream.
Show empty categories with descriptions to everyone
Add a note to allow_uncategorized_topics description
FIX: Support inline code blocks with @mentions in them.
Refactor User.find_by_username_or_email

* Improve test coverage
FIX: New lines in code blocks were somewhat unpredictable
digest email footer should say "unsubscribe"
FIX: @mentions within backticks followed by proper @mentions.
FIX: Following a mention with a coded mention.
Fix a case when the wrong topic is loaded because the slug starts with a number
Fixes @mentions inside of lists
FIX: Autolinking within a list
Log IP addresses with screened emails and urls
Banned users are not returned as pending review users
Merge pull request #1345 from salbertson/refactor-find_by_username_or_email

Improve test coverage and refactor User.find_by_username_or_email
Update client.zh_CN.yml
add "temporarily" to flagged content hidden
Extracted nickname registration into a private controller method
remove onebox info left padding