reviewboard.upgrade was written just before we began introducing type

hints. Given how important these steps are, and in preparation for

upcoming work on this module, this change adds type hints so that we

won't have any bad assumptions on data in our pre-upgrade state

dictionary.
Along with type hints, an UpgradeStateError exception class has been

added. This is raised when we fail to find a key we expect to find.
These apply to both ./reviewboard.manage.py upgrade and

rb-site upgrade.

mypy and pyright were happy.
Tested upgrading a Review Board 4 database to 5, using both

./reviewboard/manage.py upgrade and rb-site upgrade.