Notifications should be throttled to avoid sending a bunch of emails when a handful of changes are made to a feed in sequence

[landonreed]

Observed behavior

If a user is making multiple changes to a particular feed source, a separate email notification will be sent to all subscribers for each of those individual changes.

Expected behavior

Instead, we should throttle those notifications by queueing them up, waiting a few minutes before sending, and then ultimately sending off the notification with a description of all of the activity.

Steps to reproduce the problem

1. Create a feed source
2. Watch the feed source
3. Change a feed source property (like name)
4. Change another (like Deployable)
5. Observe multiple email notifications.

Any special notes on configuration used

N/A

Version of datatools-server and datatools-ui if applicable (exact commit hash or branch name)

latest dev