Add 'locale' field to JustfixUser model.

[toolness]

This helps with #1384 by adding a locale field to the JustfixUser model. This will help us when we need to initiate tasks on behalf of the user from a context where we can't retrieve their email from a request, e.g. sending them an email or text message from a worker process.

The locale field is set when a user first onboards with us, and its value is taken from the request, so if the user onboards in Spanish, locale will be es.

Note: Because Django doesn't actually set defaults at the database schema level, pushing this change to production might require us to temporarily disable Heroku Preboot, since old dynos creating new users post-migration won't know about the locale column and Postgres will demand that they set it.

To do

• When a user signs up, set their locale to the locale of the request.
• Add tests.