Make the composer and TopicCreator use category id instead of category name

Also fixes #1171
Added backwards compatibility for topic creation
PNG Files optimization
Update client.zh_CN.yml
Extracted nickname registration into a private controller method
add a site setting for allowing animated avatars
Merge pull request #1372 from ZogStriP/site-setting-for-allowing-animated-avatars

add a site setting for allowing animated avatars
Enum site settings can have translatable names in dropdown. Add setting for how often users get digest emails by default: default_digest_email_frequency.
Version bump to v0.9.6.1
FIX: Emoticons were parsing twice sometimes.
clarify on invite friends
default relative dates to 30 days vs 14
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***