The datepicker behaved weird in firefox, and I think fixing it
has no priority now. I've created the VettedThesisLinkFactory
and wrote a couple of happy-path tests for the theses view.

Change all involved tests. Start moving email to templates.

Update all involved tests. Also extend model_form_data to
accept an argument `form_kwargs` to pass to form.

Creating a new ThesisLink didn't save the `requested_by`
 field, causing trouble in for example the vet view
 for these specific objects.

Test if thesis is vetted/deleted and mail is sent in
the accepted/refused case.

Theses that are accepted are vetted by the correct contributor.
Mails are not yet sent. Thesis is not yet deleted when refused.

This does not work yet. I wrote some failing tests, but while
I was doing that, I came to the conclusion that I want to make
the route of the get request different from the route of the post
request.

Removes some 40 lines of boilerplate, since CreateView
handles the creation of a new ThesisLink.