We already had different functionality for email branding and will
soon be adding more for email branding pools.
Note that the "get_available_choices" class method was only used for
email branding - we can do it in the constructor for letters.
This was a lot of code to be in a form and it's going to get even
more complicated with email branding pools. Moving it out means we
can also simplify the tests that target this code.