CLC-2252 Switch to stable login_id as Canvas user ID

https://jira.media.berkeley.edu/jira/browse/CLC-2252

Currently sis_login_id and sis_user_id are equivalent for all our Canvas accounts. This CalCentral change must be in place before we begin changing their sis_user_id values to either-LDAP-UID-or-student-ID.