[bug 843732] Capture src in source field

[willkg]

r?

[willkg]

It's pretty straight forward. If there's a src= querystring parameter, we truncate it to 30 characters and stick it in the db.

At some later point (and under a different bug), we'll validate the src parameter against campaigns or something like that.

There's no way to view the source on the site, yet. Once someone figures out how they want to do that, we can do that on the site.

[willkg]

^^^ Redoes the PR to handle utm_source and utm_campaign.

[rlr]

I ran the tests, did manual tests with different combinations and verified the values in the admin.
r+!

[willkg]

Landed in 878db23 [bug 843732] Handle utm_source and utm_campaign