Refactor all form functionality into `common/forms` subapp

This PR refactors all form functionality into a common/forms subapp. The motivation for this is that our common app has become large and bloated, mixing together a lot of different functionality. This makes it difficult to get an overview of the functionality that exists in the app.

Should this new forms app be a "full" app, or just a subapp? I'd love to get some thoughts on this. I think more functionality currently existing in common should be moved into their own apps, to better organize our repo. However, I also feel that some of this functionality still feels very at home in a common app. Maybe this is just because I'm used to the functionality existing there? I want to move more functionality into separate apps (e.g. Markdown, breadcrumbs) and I'm not quite so sure I like the subapp concept for those.

I decided to not move create_formset_post_data() because I feel this function is more related to testing than forms.