CLC-3186: LiveUpdates: Start doing warmups earlier

https://jira.media.berkeley.edu/jira/browse/CLC-3186

Start doing warmups earlier.
Trigger warmup from BootstrapController as soon as we know we have a valid session[:user_id]
Merge pull request #2064 from chikoon/CLC-3186-start-warmups-early

CLC-3186: LiveUpdates: Start doing warmups earlier
wrote failing test to make sure expiration is not the config value
added my_up_next.derived_expiration method (not being used or tested yet)
CLC-3190: LiveUpdates: UpNext merged feed cache should expire at midnight every night

https://jira.media.berkeley.edu/jira/browse/CLC-3190

The UpNext merged feed now expires at midnight every night
instead of never expiring. Now, it will be fresh every day,
and should no longer show old data for first-time-in-the-morning logins.