Added approver notification setting

What does this MR do and why?

Adds a new notification setting for users to get notified when the are added as an approver. When a merge request gets created there is a delay to make sure that all approval rules are created and then we send notifications and create todos for users that have this notification setting enabled.

Migration up

main: == 20231114152304 AddApproverToNotificationSettings: migrating ================
main: -- add_column(:notification_settings, :approver, :boolean, {:default=>false, :null=>false})
main:    -> 0.0030s
main: == 20231114152304 AddApproverToNotificationSettings: migrated (0.0067s) =======

main: == [advisory_lock_connection] object_id: 184460, pg_backend_pid: 7274

Migration down

main: == [advisory_lock_connection] object_id: 184420, pg_backend_pid: 6776
main: == 20231114152304 AddApproverToNotificationSettings: reverting ================
main: -- remove_column(:notification_settings, :approver, :boolean, {:default=>false, :null=>false})
main:    -> 0.0044s
main: == 20231114152304 AddApproverToNotificationSettings: reverted (0.0103s) =======

main: == [advisory_lock_connection] object_id: 184420, pg_backend_pid: 6776

#12855 (closed)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

• Enable Added as approver notification setting under user notifications
• Add approval rules to a project, either code owners or normal project rules
• Create a new merge request
• Todo should get created and email should be sent after a short delay

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.