Add form to bulk-associate multiple managers with multiple groups

Currently, the Add Manager(s) form allows multiple managers to be assigned to a single group. This can lead to a lot of duplicated effort, particularly in large systems where multiple managers may be assigned to several groups.

Create a form, perhaps accessible via the Admin section, that has two fields:

- managers: multi-select
- groups: multi-select

When submitting the form, all selected managers should be assigned to all selected groups.