Implement a wider window to account for drifting clocks.

Problem/Motivation

Some users are reporting they can't login. I think this could be because of drifting clocks. See this thread on the issue: https://security.stackexchange.com/questions/113208/how-big-window-is-se...

Proposed resolution

I think storing a "drift" for each user is way too complex, so I think a reasonable buffer 5 minutes as suggested above.

Remaining tasks

User interface changes

API changes

Data model changes