Remove default DB upgrade notice and replace with custom notice

This custom notice will link to a global location and process
database upgrades for all sites on the platform rather than only
at the network level.
Update DB schema for all sites globally rather than per network
We no longer need the parent_file or current_screen globals
Upgrade site DB tables 20 at a time rather than 5
Merge pull request #192 from washingtonstateuniversity/global-upgrade

[PR] Handle database updates globally rather than per network