[server] Delete all the memberships where role is -1 and user no longer exists

Summary: I also added a function which replaces main function which we write in every script

Test Plan: I created a row with such conditions, run the script and verified that the row was deleted and no other rows were affected

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: Adrian, zrebcu411, KatPo

Differential Revision: https://phabricator.ashoat.com/D538