BE: Allow group owners to remove users from the subscription

This issue is a further iteration of Create Dynamic "Seats Currently in Use Members List" for GitLab.com

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Problem to solve

When an employee leaves a company, it can be quite manual and tedious to remove their access from GitLab if the user is a member of several groups/subgroups/projects.

Target audience

Group owner

Proposal

On the list of users occupying seats in the subscription, add a button to completely remove the user from the subscription (all groups/subgroups/project).

Potential issues

Removing a user who is the only owner (or only member) of a project may cause the customer to lose access to that project. If we decide to offer this functionality, we should make sure that this is clearly communicated to the customer.