[BE] Track member last activity time

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 logic to update Member's last activity timestamp

We can follow the same path that was taken for Users#last_activity_on for this, as it'll be used in much the same way.

The actions we will track last activity for are:

• Git HTTP/SSH activities (such as clone, push)
• Member visiting pages related to dashboards, projects, issues, merge requests, settings, etc.
• Member using the API
• Member using the GraphQL API

This list uses the User implementation as a basis, and was confirmed/defined in Define: Dormant Members (#460840 - closed).

Considerations

We should consider scale implications of this approach, as for each of these actions we will need to:

• find the group
• find the relevant Member record for the Group/User combination
• Update the Member record
• This should be behind a feature flag