In req/req/create when the site is selected, prefill the contact / requester field with an appropriate person, preferably the site contact.

See ToDos in the new .js file and in site_contact in controllers/req.py.

The autocomplete for the contact fields locks the fields after setting them. None of the other fields lock, so this seems inconsistent. I didn't lock the fields in this change, as I'm wondering if that should be made optional and then turned off for the autocomplete.

I don't get a "navigate away" warning when I refresh the req/req/create page after entering data.