shorten registration form

[glbert-does]

• rating can easily be fetched from lichess and is not needed.

• we know whether the account of a player is linked. asking if the player is on the slack group is somewhat unnecessary.

• we know whether someone was an alt in their last season or not. [Team league only]

for all 3 of the above, i believe that the information given by the user is largely not used anymore anyway. make sure that is the case. if it is still used, look into fixing that. if it is not used, the item can be removed from the registration form.