Improved waiver claim visibility

Added functionality for users to see their own waiver claims from all previous gameweeks, and to see league-wide successful transactions on the same basis.

Also made a supporting performance change, moving waiver claims in to their own database objects rather than a list within the user object. (Key to avoid late-season login performance degredation for users with many waiver claims, and also improves performance of the new views added.)

This will result in all existing waiver claims in existing implementations becoming invisible. League admin should run the new transfer_waiver_claims management function in manage.py