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
Change reject_bulk and destroy methods

Create a private method to define the new instance of UserDestroyer. The
reject_bulk method now uses the group_by method to collect all the
results of the check into a Hash. This way, it just collects all the
true values to get the successes and subtracts that number to the total
number of users for the failures.

The destroy method now uses a top-level rescue clause to make the method
a little more readable.