WIP - add speakers inline

Specify HeavySelect2Widget rather than inspecting the MRO.

I don't think it's safe to inspect the MRO to decide where to
go next, and it certainly looks more complicated, so let's
manually go to HeavySelect2Widget instead.

Test that a speech with a new speaker causes that speaker to be added.

Use the heavy versions of the django_select2 css and js.
Add test for ajax lookup of existing speakers.