Remove competition start date validation

This is really getting in the way of creating data for tests, and it
doesn't currently server any purpose, since there's no functionality to
create a competition via the app.
Store ride counts in memberships table

Adds a ride_count column to the memberships table, which keeps track of
a user's contribution to a team. Also adds logic to calculate scores on
a week-by-week basis.
Add script to update ride counts
WIP Using new calculations in app
squash! WIP Using new calculations in app
squash! WIP Using new calculations in app

Use new score logic in dashboard
squash! WIP Using new calculations in app

Fix broken logic
WIP team participation spike
WIP member participation spike