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

https://jira.media.berkeley.edu/jira/browse/CLC-2712

The original CalCentral UX design limits My Activities to items from the past 10 days. Jon has asked that the dashboard instead show everything that bCourses itself would show on the user's front page (up to 21 items, up to 4 weeks old).

Note that this change means that bCourses activity will often be considerably older than other items in the feed. If the difference is too obtrusive, we may have to trim it again.