refactor(forms): clean up FormTemplate code and make submissions, decisions and reviews all use it the same way

I'm trying to rationalise dynamic forms by making them work the same for submissions, decisions and reviews. This will allow us to reduce the complexity of export and publishing code, and also opens the possibility of having a shared document model for all these different document types.

I have also substantially cleaned up the FormTemplate, moving out a lot of logic that shouldn't be its responsibility. This will make new features such as nested forms a lot simpler to achieve.

This is a work in progress. Substantial testing is still needed on imports, exports and publishing, as well as cypress tests. File uploads in the CMS Layout are currently not working correctly.