Add import column for assigning users to manual group lists

[tgaffey]

Add support for a new GroupCode column that can contain comma-delimited and/or be repeated to list a set of pre-created group handles to assign a student to.

The feature would not support creating any nonexisting groups, because enabling it to be placed anywhere in the hierarchy via data in the import would be overly complicated. Instead, it should error out for any group values it can't lookup.

There is an existing Group/Cohort import field that optionally assigned users to auto-generated subgroups of the students/alumni tree. We'll need to avoid conflict with that one. Groups assigned via this new field should be combined with any group assigned by the current logic (which can be disabled).

Ensure that the combined group assignments list feeds into the clearGroups logic that removes students from any groups not assigned by the current import.

Through the new array column support, both of these should be easily made to work at the same time by accepting an array and splitting any string values:
Username,GroupCode "tomg","b21-staff,teachers-math" "jstudent","students-2022,teams-crosscountry,cohorts-2018"

or, this would also work:
csv Username,GroupCode,GroupCode,GroupCode "tomg","b21-staff","teachers-math","" "jstudent","students-2022","teams-crosscountry","cohorts-2018"

Original post: https://trello.com/c/cvfv9c8Z

Story points: 5