Add proper support for the new media/ext directory on installation.

RB 1.7's extension support requires a writable media/ext directory.
rb-site and prepare-dev now create it, and the manual updates checks
make sure we can write it.

Performed a new installation and upgraded an existing one. In each case,
the directory was created and I was told I needed to set permissions.

When I ignored that advice, I saw a manual updates page forcing me to
update them.