Apply constraints when deleting relay groups

[christophermaier]

No description provided.

[mpeck]

The changes look reasonable to me. But I wonder if we would want to provide a way to remove a relay group without having to first remove all memberships and assignments. Maybe like a force remove or something?

But if that's not something we want. This looks good to me 👍

[christophermaier]

@mpeck I think we wanted to avoid making big changes to the API so close to 1.0. In any event, cogctl now has some commands to help with this (e.g. cogctl relay group remove $GROUP --all will remove all relays from a group, and cogctl relay group unassign $GROUP --all unassigns all bundles from a group. If we wanted to, we could easily add a --force flag to cogctl relay group delete $GROUP without touching the API.

This change was to ensure that the API didn't throw a HTTP 500 when you tried to delete a group.