Fix move flags bumping over line numbers.

In the diff viewer, the flagpole for moved-line flags would cause a
misalignment of the line numbers. The diff viewer style had some code to
properly cope with the added border that move flags introduced, but it was only
active for replace blocks (because we used to put the flags for delete/add
blocks) on the other side of the text column. I've made it so this rule works
for all ".moved-row th" elements.

Testing done:
Loaded a diff that had some moved code. Saw that the line numbers were all
properly aligned.

Reviewed at https://reviews.reviewboard.org/r/5500/
Fix index line splitting to not cut off filenames.

The splitting for index lines would cut off any filenames that included a
space. This was due to passing '2' to split() where we should have passed '1'.

Testing done: Ran unit tests.

Fixes bug 1533.

Reviewed at https://reviews.reviewboard.org/r/5501/
Add documentation for AuthBackend.login_instructions.

Adds documentation for the login_instructions attribute of AuthBackend to the
"Writing Authentcation Backends" page.

Testing Done:
Visually inspected generated HTML docs.

Bugs closed: 3254

Reviewed at https://reviews.reviewboard.org/r/5502/
Fix retina.js with images not currently visible on the page.

retina.js has a bug (which has a few issues and pull requests filed)
where any images not currently displayed (either through display:none on
the image or on a parent) will end up with a width and height of 0.

The solution appears to be to use the naturalWidth and naturalHeight
properties instead of offsetWidth and offsetHeight. Others have deployed
this without problems, and it works for the cases I've hit as well.

The problem is that we now have to maintain this, but hopefully the next
time retina.js puts out a release, it'll include a fix.