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
Add support for lists to readability topic importing.
FIX: Uncaught promise on link click; fixes broken build
Add site_description to meta description tag for real
Version bump to v0.9.8
username characters validation error should include underscores
Merge pull request #1805 from shivpkumar/master

username characters validation error should include underscores
BUGFIX: in some proxy scenarios https was not propergated properly
don't insert middleware its a problem
update bench
update bench
Update Chinese Translation.
Fix method name in code documentation for prependMore
Merge pull request #1806 from bbs4us/master

Update Chinese Translation.