Group webhook for members (edit, remove)

Release notes

In Gitlab 13.7, we introduced a webhook that is triggered when a new member is added to a group. Gitlab 13.8 introduces additional events. The group member events webhook will also be triggered if the access level of a user has changed, the expiration date for user access has been updated or a user has been removed from the group. With the addition of these events, the group member events webhook can be used to be aware of all changes made to group members without relying on API calls which put unnecessary performance load on your GitLab instance.

Documentation: https://docs.gitlab.com/ee/user/project/integrations/webhooks.html#group-member-events

Problem to solve

Follow up issue from Group members MVC for webhooks.

Proposal

Work for events when members are added were tackled in #260347 (closed). This issue will contain events for when:

• The role of a member is changed
• A group member is removed
• An expiration date is changed for a member
• For consistency with other group-level webhooks, we should post for events that happen in the group where the webhook is set up and its subgroups and projects