Activity Feed - Saved Searches [MJ-369]

Adds the ability to filter saved searches by microsite and date. Also shows the gravatar and name, if they exist, of the owner.

Filtering is currently manual and will need to be refactored upon the introduction of the company object.

There is a test page located at /activity/ to test this. The testing user must be a member of the 'Staff' group.

Testing on production data with a very broad filter like 'jobs' and a very wide date range results in a wait of ~12 seconds but shows every search in existence. Searching for something smaller like 'indiana.jobs' or using a smaller date range results in a much shorter wait. It was my understanding that this just adds the feed functionality and the ux would be improved during  [MJ-368](https://directemployersfoundation.atlassian.net/browse/MJ-368) so this is somewhat bare-bones at the moment.

I am open to suggestions regarding the improvement of the queries.

All tests pass.