Don't use <br/> tags for newlines in Markdown text in e-mails.

We had the "nl2br" filter applied for Markdwon text, which caused every
newline to have a <br/>. However, these appeared inside a <pre/>, and we
still had newlines. The end result was that we had a blank line for
every line of text.

I can't see any reason why we'd need this filter, so I've removed it.
This results in proper-looking e-mails.

Reviewed at https://reviews.reviewboard.org/r/6286/
Fix URLs to users on Local Sites.

The URLs shown in the user infobox, in the Submitter field on a review
request, and in the target reviewers lists weren't correct on Local
Sites. We were using get_absolute_url, and need to instead use
local_site_reverse.

After this change, all URLs to users should be correct.

Reviewed at https://reviews.reviewboard.org/r/6289/
Fix the display of the floating review banners while scrolling.

The floating review draft banners had some issues when scrolling off the
edge of a review. Due to some imprecise calculations, the page would
appear to stutter for a brief moment while scrolling, as the banner
jumped back and forth between being docked to the page and floating over
the page.

To fix the stutter, we needed to set the placeholder to have the exact
height and margins of the original banner, pre-float. This kept the
container the same size. We also needed to fix some of the calculations
to not go in/out of float mode too soon.

With this fixed, the page no longer jumped, but the banner would just
disappear as we hit the edge of the review. Now, the banner will appear
to dock at the bottom of the review, and just scroll out of view as the
user scrolls further down.

The end result is a buttery-smooth scrolling experience.

Reviewed at https://reviews.reviewboard.org/r/6293/