Add errors related to "leave team" feature

[smcmurtry]

Summary | Résumé

This PR:

• adds a method on the service object get_users_with_permission - this returns a list of active users who belong to the service who have the permission you pass in as an argument
• adds 2 new error responses that are returned when you try to delete a user. SERVICE_CANNOT_HAVE_LT_2_MEMBERS and SERVICE_NEEDS_USER_W_MANAGE_SETTINGS_PERM. This is to support the "leave team" feature in notification admin

Related Issues | Cartes liées

• https://app.zenhub.com/workspaces/notify-planning-614b3ad91bc2030015ed22f5/issues/gh/cds-snc/notification-planning/1158

Test instructions | Instructions pour tester la modification

1. run this branch locally
2. run the feat/remove-yourself branch of notification-admin locally
3. set up a service with 2 members, one member with all permissions, and the other member missing the "manage settings and team" permission
4. click "leave team" from the "team members" page, confirm, and observe that you get an error about not being able to leave a team with only 2 members
5. add another user to the team who does not have the "manage settings and team" permission, bringing the total to 3 team members.
6. from your user account who does have "manage settings and team" permission, click "leave team" and confirm. You should now see an error about the service needing at least 1 user who can manage settings and team

Release Instructions | Instructions pour le déploiement

None.

Reviewer checklist | Liste de vérification du réviseur

• This PR does not break existing functionality.
• This PR does not violate GCNotify's privacy policies.
• This PR does not raise new security concerns. Refer to our GC Notify Risk Register document on our Google drive.
• This PR does not significantly alter performance.
• Additional required documentation resulting of these changes is covered (such as the README, setup instructions, a related ADR or the technical documentation).

⚠ If boxes cannot be checked off before merging the PR, they should be moved to the "Release Instructions" section with appropriate steps required to verify before release. For example, changes to celery code may require tests on staging to verify that performance has not been affected.