Add a manual update check for static media configuration.

Review Board 1.7 requires new static media configuration in the web
server config files. As things currently stand, anyone who upgrades will
see their media broken.

Now, any new upgrades from a database prior to 1.7 final that hasn't had
this already resolved will cause a flag to be set in the database
indicating that static media needs to be fixed. rb-site will output
instructions, and the manual updates page will list instructions as
well. Both sets of instructions will say what to type to resolve this.

Resolving the manual update check is done by using the new resolve-check
management command.

This should ensure that all users who upgrade to any release post-1.6.x
will see instructions and have to act to make things work again.

Upgraded an older release to 1.7 alpha. Saw the upgrade instructions printed
on the console. Verified the flag was set to False (unresolved) in the
database.

Went to RB and saw the manual updates check.

Ran the command given to resolve the check, reloaded, and I was able to view
the site again.