Allow using extension management commands in rb-site manage.

rb-site manage was loading the site configuration but not initializing
all of Review Board. This meant that any custom management commands
couldn't be used. We now initialize Review Board so that all extensions
are also loaded.
Require the extensions static directory.

The manual server update checks now look for the extensions' static
directory along with the old legacy directory, and require it to be
writable by the web server before Review Board can load.

rb-site now creates this directory on install or upgrade. We still don't
set the permissions, but instead tell the user to do this (after
install).

We're also removing the direct Pipeline dependency, since Djblets
depends on this now.
Remove the GTK+ wizard for rb-site install.

This gets rid of the GTK+ wizard. Since we don't do much work on Linux
anymore, and most servers will be accessed over a shell, there's not a
lot of point in keeping this. It'll just stagnate and make development
harder.
Skip dimmed whitespace chunks in keyboard navigation.

When "Hide whitespace changes" is selected, changes to trailing whitespace are
shown as equal instead of a replace. Unfortunately, the next/previous keyboard
shortcuts were still stopping to highlight these chunks.

This change fixes up _getNextAnchor to take into account whether or not the
parent chunk of the anchor is dimmed or not.

Testing done:
Created a diff with some whitespace changes, hid those changes, and then
browsed around with the keyboard. Made sure that it worked as expected when the
changes were shown.

Fixes bug 3280.

Reviewed at https://reviews.reviewboard.org/r/5640/