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
Fix inviting others to private messages.
Merge branch 'ubuntu-install-notes' of github.com:bloomberg/discourse into sysadmin-docs

Conflicts:
	docs/INSTALL-ubuntu.md
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
Merge pull request #1849 from vikhyat/fix-pm-invite

Fix inviting others to private messages.
Merge pull request #1850 from Supermathie/sysadmin-docs

Finally merge in @kpfleming's suggestions in #1430
Merge pull request #1841 from CiderCooler/master

Clarify commands around rvm
FIX: Updated Markdown-js to include my latest FIX from upstream:
https://github.com/evilstreak/markdown-js/pull/164
FIX: Deprecation warning on bindAttr
Fix cold colours in activity column of topic lists
FIX: keyboard navigation in the search box results dropdown