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.