Allow a default callable to be passed to none_to_string
Replace None with empty string in org form.
Converts Nones to strings before rendering membership form.
Remove all underscore fields from memberships before validating.
Add default args to none_2_string calls
In obj_as_popolo method, remove internal underscore fields before validating; add them back in afterwards.
Better comments in persom-membership creation function
Made the new membership share the sources of the new person.
Temporarily copy obj_as_popolo function from people; will move to common location later.
Avoiding unsetting fields that don't appear in the form
Use helper function to validate form data and preserve udnerscore fields.
If form is invalid on creating person, don't try to pull _id data from form.
Merge branch 'master' of github.com:opencivicdata/anthropod