CLC-3187: LiveUpdates: Do warmup in a forced-cache-write mode (Part 2)

https://jira.media.berkeley.edu/jira/browse/CLC-3187

UserCacheWarmer now calls each model's get_feed_as_json method with the forced-cache-write parameter so that the cached json objects get warmed up, too.

Before this change, the cached data wasn't being warmed when up_next was called from the UI.

Thanks @chris for your help on explaining this to me.