Tri-form

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* Add `raw_data` and `raw_data_list` attributes to `Field` definition to enable custom handling on django file download

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* Set instance on form just after save to be able to use newly created pk in a redirect

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* Prevent code from running on import

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* Lazy evaluation of select choices.

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* Add `endpoint` namespace to `Form` and `Field` constructor to add custom endpoint callbacks.

* Add `config` and `validate` endpoint implementations

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* Fix formatting of form field error output when there are multiple field errors

* Fix bug in form validation signaling required field missing on choices where the value is falsy, e.g. 0

* Implement djangos uniqueness validation on save in forms/views.py

* Fixed input type on url field