Adds datefield support with flatpickr

[saschalalala]

Closes #72

I should have commited more …

Well, I'll quickly explain what this PR does.

forms.py

Same as in #86 but with an additional css class for date questions. This is required for the javascript library to run on the respective input fields.

question.py

Same as in #86.

survey_detaily.py

I didn't want to do these checks inside the template(s), so I introduced a new context variable that can be used to control asset loading, because I think, it is a nice feature to only load flatpickr js and css if it is really required on the current page.

Templates

Asset loading and some indentation fixes.

Note that I also added two new static files. In order to commit them, I had to disable the pre-commit hook, but I ran the tests and flake before.

Also, the same "problem" as in #86 regarding the translation exists, because I didn't understand yet, what you did there. :)

[Pierre-Sassoulas]

Looks good! In order to fix the translation, you can launch the tests locally with python manage.py test. It launches a normalized makemessages that you could launch yourself and that is probably defined somewhere in the readme.md. Then you use git add survey/ -p and add everything except the changed date in the .po file. (The changed date is nice in theory, but in reality always just make the rebase nastier)

[Pierre-Sassoulas]

Very good work, thank you !