Update max age expected migration

This is not a good way of doing this. This is no longer a migration,
it's a scripted way of keeping the metadata up to date. This will be
better when it's managed by stagecraft.

- Recalculate max_age_expected for every bucket, only update the bucket
  if this has changed.
- Set all housing and student-finance data groups to None
- Set a couple of extra specific buckets to None
- Change how the max age is calculate, first try by bucket name, then by
  data group, then by data type, then default to 1 month.
Merge pull request #243 from alphagov/update-last-time-expected-migration

Update max age expected migration
WIP