Users should only be able to join leagues that match their rank

[paulgosnell]

All users have a rank and all leagues have a rank so we need a function that covers a few scenarios;

1. users should only see leagues that match their rank on the all-leagues screen, so when fetching leagues, a filter is needed
2. if a users rank changes whilst they are competing in a league, we notify them to rank up within a 1-week grace period, and if not, they get ejected