Begin to add validation for create/edit views

[xmedr]

Overview

This begins to add validation for create and edit views. This is done through model validators for all fields except geometry, where we're instead catching exceptions raised in both LicenseForm in forms.py and a custom widget in licenses.py

Connects #41

Notes

This does not include the items in #41 that needed updated rules, like township or job_number fields. Any other fields not mentioned in this comment should be good to go.

The fields that do have new validation should still accept existing values with little to no modification.

Testing Instructions

• Create/edit a few types of documents with incorrect and correct inputs
• Confirm that:
  • Proper errors display and the document is not saved when incorrect
  • The document is saved when inputs are correct

[fgregg]

looking good. the int checks don't look like they are actually testing ints?