Also fix missing template error for modified thesis links.

The Comment model now has a option to
 append a file (pdf, png, jpg) to itself.

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.

Improve the templates using template tags.
 Further, wrapping the `content` block in the
 `container` div element.

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.

Work in progress. All unvetted theses are now listed at
theses/unvetted_thesislinks. Vetting editors click on
a thesis link to vet, and are redirected to
theses/vet_thesislink/<id>, where they can change fields of a thesis
link.

theses/vet_thesislink/<id> is now a regular update view. It does not have
mail capacity, or refusal reasons, etc. I will work on that next.

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

Markup logic should be in templates, not in model layer.

Partial template is named _header_as_table to signal
that it is ment to be included into other templates
(analogous to the private method naming convention.)
The full text of several of the thesis link's fields
(domain, subject_area, etc.) are fetched from constant dicts
in custom filters defined in theses.templatetags.theses_extras.

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.

Move few constants to a seperate file away from models.
This implies minor edits in other apps relying on
these few constants.

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

This function helps to create form data from models.
See example given.

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

A CreateView class based view should replace the current
request_thesislink function based view, since it saves
many lines of code and is less error prone and more easily readable.

You can now just call ContributorFactory() to get a freshly instantiated
Contributor to be used for testing purposes.