Hide deleted posts by default for staff

2-month old meta post: https://meta.discourse.org/t/collapse-deleted-posts/13942

This is a very big quality-of-life improvement for admins.

Notable pieces:

 - 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 these weird percentages in the CSS are...weird
 - 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

Future improvements:

 - Style gaps with only deleted posts differently

The screenshot below shows the added URL parameter, which is 'discoverable' via clicking avatars in the topic map. It does *not* show the gap ("1 post hidden") width CSS change.

![Screenshot of 'Hide Deleted Posts'](https://slack-files.com/files-pub/T0250P76L-F02DJQVQ7-9c31e9/pasted_image_at_2014_07_15_12_13_pm.png)