Prepare Member model for committee handover

[boardfish]

We'll want previous records of committee preserved, and we'll also want to rotate a new committee in once the old one leaves. There is also potential for one member to take multiple roles, so the Member model should have more fields added to bear this in mind. We may need a Committee model that acts as a group of Members – the Committee page on the site should then display the last created Committee/a Commitee of an Admin's choice depending on how ready the new Committee is.

[boardfish]

Very important now that handover's almost here.