Added very basic Open Forms integration

[joeribekker]

Very basic integration of Open Forms building on top of a detail page to show the form directly how it would show in Open Forms. This works with CSP headers and you can nicely select a form on the product admin page, once you configure Open Forms client in the admin.

Make sure you configure the environment variable "OPEN_FORMS_DOMAIN" to activate proper CSP headers. I didn't see any place to document this environment variable

Improvements can definitely be:

• Tests that adding a form actually results the form page being rendered with the expected snippet.
• Better page layout to show the full form.
• Use of the future 0.1.1 library django-open-forms-client because now there is a default UUID in the migration that I don't want there.
• Optionally, you might want to set the CSP-domein via database settings. We already add the domain in the OpenForm Configuration model, so might hook into that.

[joeribekker]

That black failure is on the develop branch btw. but didn't want to touch it.

[vaszig]

• As discussed with Joeri as well, this shoud not be merged until it is updated with the newer version of open-forms-client.
• The domain (OPEN_FORMS_DOMAIN) should be added via an environmental variable for now.

[vaszig]

As discussed, the side bar {% block sidebar_content %} can be removed and add only a "back or cancel" button, but this can be done when we improve the layout generally.

[joeribekker]

Updated the lib and this PR. No default uuid in the migration needed anymore.

[vaszig]

I think something goes wrong with some forms concerning the max length of the SlugField of Django. Is this happening to you as well?

[joeribekker]

Weird, I thought slugfields had a length of 100? I'll check it out. I specifically used the same field as Open Forms to prevent exactly this problem, sigh :)

[joeribekker]

Increased length to 100 chars. Note, you need to rollback and migrate again to make this effective.

[vaszig]

Tested with the 0.2.2 newer version and it works fine!