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.

Removed both the directories, and saw the manual updates warnings.
Created a site and saw the static/ext/ directory. Removed it, upgraded,

and saw it get rebuilt.