Various cleanups to Ubuntu installation instructions:

* Avoid editing /etc/apt/sources.list add nginx repo, instead create
  /etc/apt/sources.list.d/nginx.list

* Avoid editing /etc/nginx/nginx.conf, instead put server_name_hash_buckets
  setting into discourse.conf

* Proper-case Discourse when it is referring to the product name, so that all
  the lowercase references are to the user created during the installation
Address @Supermathie's concerns in PR1430
internationalize Discourse.Formatter.tinyDateYear
Allow groups to be used as aliases for user mention

when configured by the admin a group can be found through the @mentions
feature in both the compose/reply and the private message user-selectors
and once selected the mention will be replaced by the list of users in
the group
move alias levels into enums
internationalize Discourse.Formatter.tinyDateYear
   missed client.??_??.yml
Fixing padding for misaligned small logo URL
Fix method name in code documentation for prependMore
Update Simp Chinese (zh_CN) translation #2.
Update Simp Chinese (zh_CN) translation #3.
Refactor guardian as dissused in this topic https://meta.discourse.org/t/so-you-want-to-help-out-with-discourse/3823/41?u=hunter

Creates a mixin for the ensure_* functions and creates seperate mixins for functions dealing with posts, categories, and topics.
Update Simp Chinese (zh_CN) translation #4.
Merge pull request #1815 from PatrickHunter/master

Refractor guardian as dissused in this topic https://meta.discourse.org/t/so-you-want-to-help-out-with-discourse/3823/41?u=hunter
Merge pull request #1813 from bbs4us/master

Update Simp Chinese (zh_CN) translation.
Merge pull request #1807 from balinterdi/patch-1

Fix method name in code documentation for prependMore
Merge pull request #1795 from ok-harry/master

internationalize Discourse.Formatter.tinyDateYear
Merge pull request #1782 from ligthyear/group-mention

Allow groups to be used as aliases for user mentions
BUGFIX: Improve copy and correct instruction alignment
FEATURE: query param for closed / archived / open

?status=closed for closed topics
?status=open for open topics
?status=archived for archived topics
Clean up Mac OS how-to, add troubleshooting step