PNG Files optimization
Update client.zh_CN.yml
Extracted nickname registration into a private controller method
Refactors UserEmailObserver to improve Code Climate score

- Extracts certain logic to private methods and remove unnecessary
comments
- Extracts email enqueueing methods into a separate class
- Fix specs involving UserEmailObserver to call #after_commit instead
of the specific methods
Clarify install doc wrt how to create databases
Merge pull request #1374 from manzoid/install-doc

Clarify install doc wrt how to create databases
Merge pull request #1370 from ofgeek/patch-2

Update client.zh_CN.yml
Merge pull request #1369 from voisintotoro/voisintotoro-branch

PNG Files optimization
Merge pull request #1332 from locks/user_email_observer

User email observer (Code Climate improvements)
Added strings

Added some strings not yet existing in the "es" locale but already in use in the application.
Support for ***bold and italics***
FIX: Code blocks were trimming leading spaces sometimes when they should have only
trimmed trailing spaces.
Extract ListController#list_target_user
Remove enum setting spec since I don't think it's testing anything useful and it's causing random failures
FIX: Don't onebox @mentions
FIX: If a topic/post cannot be oneboxed, don't swallow the href.
improved specs for avatar
Show Private Messages pill as a topic list rather than individual messages.
add a warning whenever the uploaded image is not a square
Lower Complexity of UsersController
https://codeclimate.com/github/discourse/discourse/UsersController#method-complexity