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
Merge branch 'ubuntu-install-notes' of github.com:bloomberg/discourse into sysadmin-docs

Conflicts:
	docs/INSTALL-ubuntu.md
Merge pull request #1850 from Supermathie/sysadmin-docs

Finally merge in @kpfleming's suggestions in #1430