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
Clarify commands around rvm
Simplify example, remove typo
fixes unit test broken (and commented out) after adding debounce behavior to a method
FIX: topics with no replies should show coldness in activity column
Add the Lounge category, an exclusive area for trust level 3 users
New API for replacing elements in the final JsonML. Also changes spoiler
tag handling to be more robust with repsect to HTML content.
FIX: Remove failing ruby tests. The same thing is tested in JS and
passes due to the ability to create a document fragment.
Revert "Add the Lounge category, an exclusive area for trust level 3 users"
Add the Lounge category, an exclusive area for trust level 3 users
Changed the `topics_per_period_in_top_page` default to 50

100 was still too much
FIX: Adding components to a ContainerView doesn't set the context
properly.
Merge pull request #1846 from velesin/admin_site_settings_controller_test_fix

fixes unit test broken (and commented out) after adding debounce behavio...
Change user update to use #fetch for defaults

This changes the use of the || operator when setting the default of the
user attribute if the key does not exist to #fetch. This will make sure
that if false is passed in as the value it will still fail and use the
default setting.

This also adds constants to define and array and a hash of symbols which
is iterated over to set the users attributes.
BUGFIX: add [PM] string to private message emails
Fix inviting others to private messages.
Merge branch 'ubuntu-install-notes' of github.com:bloomberg/discourse into sysadmin-docs

Conflicts:
	docs/INSTALL-ubuntu.md
Merge pull request #1847 from nickborromeo/refactor-user-updater

Change user update to use #fetch for defaults
BUGFIX: make the system_user an elder (TL=4)

Otherwise it won't be able to create topic when the `min_trust_to_create_topic` is > 0