CLC-3189 - LiveUpdates: Eliminate the "checking for new data" prompt

https://jira.media.berkeley.edu/jira/browse/CLC-3189

Eliminated the "checking for new data" prompt that used to appear when a user's dashboard was loading.

The message was considered to be "dishonest" since it didn't necessarily always get the new data because of server caching issues.
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 data wasn't being updated when up_next was called from the UI.

Ammended: get_feed_as_json needed to pass the force_cache_write param through to the get_feed call so that the get_feed contents refresh.
Merge pull request #2079 from chikoon/CLC-3187-forced-cache-write-warmup

CLC-3187: LiveUpdates: Do warmup in a forced-cache-write mode (Part 2)
CLC-2712 Show the entire Canvas recent-activities feed on the dashboard
Merge pull request #2078 from chikoon/CLC-3189-rm-prompt-checking-4-data

CLC-3189 - LiveUpdates: Eliminate the "checking for new data" prompt
The user_cache_warmer test broke because get_feed was getting called twice instead of once.  This was because the UserCacheWarmer was changed to call model.get_feed_as_json, in addition to get_feed, and the previous stub of get_feed was getting called twice instead of once.  Revised this test to make it clear that both get_feed and get_feed_as_json are each called only once by the UserCacheWarmer.
Merge pull request #2083 from chikoon/CLC-3187-forced-cache-write-warmup

CLC-3187: LiveUpdates: Do warmup in a forced-cache-write mode (Part 3)
Merge pull request #2081 from raydavis/raywip

CLC-2712 Show the entire Canvas recent-activities feed on the dashboard