When adding evolutions for oauth2_provider I got a little lazy and just

ignored the RunPython stuff in their migrations. It turns out that we

actually need to populate the token_checksum field in the same way that

they do, first creating the field as blank, populated it with hashes of

each of the items, and then finally fixing it to be non-blank.
This also adds a post-upgrade step to save application client_secret

fields, causing any unhashed items to be fixed.
Finally, I've set a couple additional flags on the conversion of the

token field to be a TextField, removing the max length and unique

parameters.

Did test upgrades from 4.0.6, 5.0.7, 6.0.2, and 7.0.4 schemas.

Previously I was getting failures on the 4.0.6 and 5.0.7 ones.