Add choices to custom data fields

Waiting on [QA](http://manage.dimagi.com/default.asp?146254)
This is currently on staging (https://staging.commcarehq.org/a/ethan-test-domain/settings/users/commcare/fields/)

I've added the ability to specify a set of choices for custom data fields.  Currently this applies to both custom user data and custom location data.  This is live on staging.  Here's the new functionality:

Add "Choices" column to "Edit X Fields" for configuring choices.

For fields with a choice configured, the "Create a new user/product" and "Edit an existing user/product" UIs should now show a dropdown allowing the user to select from the choices configured.  Required fields with choices should force you to make a selection.

There is now additional validation on excel import - any field with choices configured should verify that all values for that field match the available choices.  An empty cell should only be acceptable if the field is not marked as required.

@twymer