Gate access to historical accounts

We only want to allow historical accounts for certain roles. This
adds a flag to the Roles model that allows us to control the roles
historic accounts can be created for. It also has a data migration
to enable historical accounts for the Prime Minster and Chancellor
roles.
Merge pull request #484 from alphagov/gate-historical-accounts-access

Limit the roles that can have historical accounts

Bug fix for: https://www.pivotaltracker.com/story/show/45246029