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.

Also fix missing template error for modified thesis links.

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.

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.

I keep get weird bugs. Maybe merging in development will help.

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.

The acknowledgement message is no longer hardcoded in
theses/views.py, but resides in the top-level strings module.

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

test_forms.py, test_models.py, test_views.py, as per
recommendation of the two scoops book.