Separate the team validation check into two parts, one for the players in the team and a second for other
characteristics of the team.
Introduce a new column for teams called "never_validated".  When true, arbitrary changes to a team are allowed, not limited by drop_available.
After the one allowed drop in a period, a user will want to add another player to complete his team.
To prevent him adding a player which would result in an invalid team (which he'd be stuck with), the last add is pre-checked for validatability using the first part (only) of the validation check described above.

Fixes #2