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.