FEATURE: Hide deleted posts by default for staff

Really old meta post: https://meta.discourse.org/t/collapse-deleted-posts/13942

This is a very big quality-of-life improvement for admins.

Notable changes:

 - A notification shows below the topic map if deleted posts have been hidden
 - **Filtering to a user will turn off deleted post filtering** unless you turn it back on
 - Fixed the width on the post gap bars. @awesomerobot this
 - Moderators can see deleted *topics* that they would be able to see if the topic wasn't deleted
 - All of this is invisible to non-staff

The screenshot below shows the added URL parameter, which is 'discoverable' via clicking avatars in the topic map:

![Screenshot of 'Hide Deleted Posts'](https://slack-files.com/files-pub/T0250P76L-F02DJQVQ7-9c31e9/pasted_image_at_2014_07_15_12_13_pm.png)