CLC-848 Warm up the user cache on login or auth token acquisition

JIRA: https://jira.media.berkeley.edu/jira/browse/CLC-848

This uses Celluloid to asynchronously warm up the cache of user-specific merged models -- basically the models responsible for the mystatus, myclasses, mygroups, mytasks, and myupnext feeds. 

There's more to do on this feature, particularly in the area of pooling the workers and properly cleaning them up, plus dealing with exceptions.