Fix bug 1080066: Add per-locale stewards and fix email field.

Adds a new model for tracking per-locale stewards for a given interest. Product_details was used for the locale field, as the locales in it should match up with the locales on Bedrock (even if Bedrock doesn't use all of them). To remove the need to run product_details on the server or during dev setup, an initial import of the JSON files is included, similar to what playdoh does.

Removes the magic in CommaSeparatedEmailField that converts the value
to a list, as this was screwing up the text inputs in the admin.

Also uses a text input for the fields instead of a textarea.
The longest locales I'm aware of in our ecosystem are Serbian variants (sr-Latn and sr-Cyrl), so 7. Not sure how relevant it is.