Make admin activity widget date range parsing a little bit more robust.
It was possible to cause the activity widget in the admin to generate requests

to the server that would generate a 500 response. The issue here was that we

didn't properly sanitize dates unless both the start and end range were

provided.
I've changed it to convert the request parameters to datetimes individually

instead of if they're both necessary, and changed the if/elif/etc block to

verify that the necessary data was provided in request.GET before trying to use

it.

Checked out the admin UI a bit.