Fix/bugs from testing

[EdoStorm96]

Let's keep this ball rolling!

So this covers most of the bugs found from testing. The only one left is regarding the stepper colors ...

#739:
So this was an oversight on my part (you might notice a pattern in this PR ... ;p) But yeah, some pretty funky functionality. I had to make sure wmo.js worked correctly and supply some ok-looking styling from check-wmo for the resulting little message. Seems to work fine now.

#742:
Ensure hierarchy gets validated.

#749:
Another oversight on my part. The PR (#715) where I implemented this stuff featured an almost identical bug, which you pointed out ... But I didn't bother to check if I had made that same mistake again (if not cleaned_data["{field}":) which becomes buggy when validating falsey values. Woopsiee. Won't make this mistake again!

#748:
Did some style stuff on the warning for "you forget to make tasks/sessions" ... Nothing crazy. However ... I did run into an annoying issue. On SessionEndForm/session_end.html, there is a non_field_error provided, which is mostly used for validating in the stepper... SessionOverviewForm has a similar purpose. However, SessionOverviewForm, does not get rendered at all. On SessionEndForm, we do render this form, so this results in an ugly duplicate warning. I was figuring out a way to remove this. Got a bit annoyed and brute forced my way through with a script tag on the template, removing the warning by class name. It's very hacky, but it works for now ... Maybe you have a better solution?

#741:
So the WMO_application page now works as expected. This is also due to a tweak of depends_on_value, where it can now also clear radio input that were dependant on an earlier question. I've gone through every use of depends_on_value and this change does not cause any issues anywhere.

I've also just pushed some other minor stuff. like using get_intervention on the StudyOverview and a minor translation fix.

The final thing is a bit bigger: I've converted StudyDesign to a normal ModelForm, to ease validation of this. It was a bit annoying and caused some of the Stepper issues ... This did require a custom rendering of this form, which is pretty crazy. But the backend stuff is much simpler now.

Happy reviewing :)