[CLC-1448] Ignore canvas entries older than 10 days, merge notifications into activity feed

JIRA: https://jira.media.berkeley.edu/jira/browse/CLC-1448

[CLC-1448] Smash notifications results together with activities, clean up endpoints & dummy data

This only deals with some of the backend items:
1) Ignore canvas entries older than 10 days old.
2) Merge notifications feed back inside activities.

Filtering will still be done on the front-end and will be additional follow-up pulls.