Notify the user of save accommodations

[ZacWarham]

It would be a nice QOL feature to have a notice once you click the Save accommodations button to let you know the save has completed. Perhaps like the green bar you see at the top of PL pages when inviting students?

[nwalters512]

Note for whoever ends up implementing this: we should strongly consider pulling the server-side sessions machinery from PrairieLearn into PrairieTest, which makes it much nicer and low-friction to add such flash messages to pages. See https://github.com/PrairieLearn/PrairieLearn/tree/master/packages/session and https://github.com/PrairieLearn/PrairieLearn/tree/master/packages/flash.