go back to SHA-256

This will eliminate the need to change CHAR to VARCHAR for the token column in the database. We now believe the issue has to do with a failure to open port 80 in the border firewall as apparently Google Calendar will try to get a robots.txt file via HTTP even if the feed is served over HTTPS. At least, that's our current best theory. Will find out for sure when they open the campus firewall. Until then, good idea to roll back changes, I think. Get rid of confusion in the code comments and algorithms about the source of the problem etc.