989932: changeset: destroy changet_user when destroying changset

Without this change an error like the following will be generated in the logs:
    ERROR: update or delete on table "changesets" violates foreign key constraint "changeset_users_changeset_id_fk" on table "changeset_users" DETAIL: Key (id)=(23) is still referenced from table "changeset_users". (PGError)
Merge pull request #3044 from bbuckingham/989932

989932: changeset: destroy changet_user when destroying changset