Rewrite webapp forms in pure django

Instead of designing HTML forms directly in django templates and using javascript to submit them, we should rather use django forms as it will enable a better interaction with associated models and get rid of javascript processing in favor of Python.

In order to implement flexible django forms in terms of design and layout, we should use django-crispy-forms as it seems a great extension and is packaged in debian.

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Migrated from T4537 (view on Phabricator)