adds is_feast_admin field to User model

There are essentially 2 types of users in the system: a regular user
and a 'Feast Admin' -- the only difference being a true value for the
is_feast_admin boolean field.

At the time of this commit, the difference in abilities is that a 'Feast
Admin' will have all the abilities of a regular user, but also some
CMS access, and user administration access