Add customer relations group setting

What does this MR do and why?

Related to #346082 (closed) and #2256

This MR introduces a new group setting to enable/disable customer relations (CRM).

Screenshots or screen recordings

How to set up and validate locally

0. Enable :customer_relations feature flag for a group via the rails console
1. Visit a group settings page (e.g. http://gdk.test:3000/groups/flightjs/-/edit
2. Note the lack of a "Customer relations" item on the left hand navigation menu/sidebar 
3. Expand "Permissions and group features"
4. At the very bottom, select "Enable customer relations"
5. Select "Save changes"
6. Note the presence of a "Customer relations" item on the left hand navigation menu/sidebar 
7. Check you can access the customer relations -> contacts and organizations lists
8. Create a contact
9. Attach the contact to an issue using the "/add_contacts email@example.org" quick action
10. Switch the feature off again
11. Check you cannot access the customer relations -> contacts and organizations lists
12. Check the contacts widget no longer appears in the right hand sidebar

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.