Refactor activate service and add an audit event

What does this MR do and why?

This MR adds a new service for the user activate action. It appears all other actions corresponding to the user model have their own service. This MR follows a similar convention and adds a new service for the activate action.

Additionally, an audit event is logged when the user is activated.

• Issue: Add user state changes to audit events (#13473 - closed)

Screenshots or screen recordings

How to set up and validate locally

1. Deactivate a user account and activate the same user - notice that the audit event is not logged within the master branch
2. Checkout the feature branch
3. Deactivate the user and activate the user
4. Navigate to the Instance level audit events page and notice that the audit event is logged for the deactivate action.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.