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
updated spanish translation
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
UI: add calendar icon to all top buttons
do not log sensitive commands during backup/restore
BUGFIX: require net/stmp module to properly handle exceptions
ooops. I don't know how to write smtp
add tests for ListController.best_period_for
German translation updated (client)
FEATURE: Warn when reviving a topic that has been inactive for X days. Setting warn_reviving_old_topic_age controls when the warning is shown. Set it to 0 to disable this feature.
Remove an unused variable in check_reviving_old_topic spec
Support for removal of old backups automatically via a site setting
Rename nickname to username in the code. Use new hub routes. (Old routes still exist as aliases for old Discourse instances.)
FEATURE: New site setting to enable a daily automatic backup
FIX: pin/unpin button in topic admin menu needs to ignore whether the admin user cleared the pin
FEATURE: Upload backups to S3 when complete.
Revert "Merge pull request #1896 from nickborromeo/category-list"
Adding name to the list of uneditable items in preferences UI

  * If enable_names,  enable_sso, and sso_overrides_name settings are true.