[FE] Add dormant member Group setting

Background

In [Gitlab.com] Automatic removal of dormant members (&7533) we intend to add a feature to identify and remove dormant users from Namespaces for GitLab.com

This is similar to the existing SM feature ( &5519 (closed)), but rather than deactivating users, we will remove members.

To achieve this, we will be adding a way to track member’s last activity timestamp within a namespace, so that we can identify dormant members and remove them if they've been dormant for greater than a predefined time period (e.g. 90 days).

Purpose

The purpose of this issue is to:

Add frontend form changes for updating group setting

The backend changes will be handled in:

• [BE] Add dormant member setting to Groups (#461334 - closed)
• [BE] Allow updating of Group setting (#461335 - closed)

We can develop this behind a feature flag before the related work is complete

Implementation

• Backend: Add remove_dormant_members settings to namespac... (!153099 - merged)
• Frontend: Allow Groups to update remove_dormant_members (!153118 - merged)