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
Refactor find_relevant_topics and find_categories

This breaks down two private methods into smaller methods for
readability. This also moves logic into the Category model class.
Merge pull request #1896 from nickborromeo/category-list

Refactor find_relevant_topics and find_categories