Custom forms

This is a pretty straightforward move from putting form definitions in the jsonforms library `ddoc.views.lib.jsonforms` to `ddoc.app_settings.forms`.  This doesn't completely fulfill #283 but is a major part of it.

Before we merge this we probably want a way to upload a file with forms in it... otherwise your average alpha user can't load forms.
Maybe expose a function from the app-settings package which does this request?