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
Delegate :username rather than using a class method.

This is just a little cleaner?
fix for nil session in mobile_detection.rb
DIEEEE SOFTWARE.MD DIEEEEeeEE
FIXES #1833, thanks @stefanpenner
Trust Level 3: invited users start at default_invitee_trust_level + 1, unless default_invitee_trust_level is 2 or higher
In development mode, if an avatar is missing just serve up a placeholder
Renames `unboundDate` to `rawDate` because `date` is current unbound.
FIX: Incorrect comment, thanks @ZogStrIP
rename `date` to `unboundDate` because it is unbound.
FIX #1831: Dates were unbound on the profile page.
Merge pull request #1836 from nitper/patch-2

fix for nil session in mobile_detection.rb
Merge pull request #1826 from nporteschaikin/patch-1

Delegate :username rather than using a class method.
Update ember-cloaking to remove dependency on I18n
Non-staff users may not delete their posts in archived topics.
sub space for underscore in 'name' when registering public folder in plugin
Merge pull request #1837 from shivpkumar/master

sub space for underscore in 'name' when registering public folder in plu...
Update client.zh_CN.yml
Merge pull request #1838 from ofgeek/patch-1

Update client.zh_CN.yml