Boundary 'start' and 'ends' dates

[symroe]

It would be useful to store the 'start' and 'end' date of a boundary.

For example, when forward-dated boundaries are imported, the date that the boundary comes in to effect is useful to know. It's also useful to know that a particular boundary will 'end' just before the new ones 'start'.

As a starting point this could just be stored on the Area or Geometry models (I'm not sure how they are related as the boundaries change, but I expect Area is the better model), but in future it would be nice to be able to query based on a date, for example, what boundary will postcode X be in on date Y.