Validate and bound ranges in massCreateTeam in teams.js

[alopezlago]

massCreateTeams should put an upper bound on the number of teams it will try to create. It should be <500, since Discord has a 500 channel limit. createTeam will probably throw when it reaches that limit; otherwise this lack of validation could be a denial-of-service risk since you could make the loop run for a long time (e.g. 9999999999... iterations).