Add support for contacts autocompletion

What does this MR do and why?

It adds support for contacts autocomplete, under a feature flag (:contacts_autocomplete). New syntax [contact:email@example.com] is used.

Screenshots or screen recordings

How to set up and validate locally

1. Enable feature flags:

Feature.enable(:contacts_autocomplete)
Feature.enable(:customer_relations)

1. Visit any group general settings http://127.0.0.1:3000/groups/flightjs/-/edit
2. Enable the custom relations in Permissions & group features (last item in the section)
3. Check if you have any existing contacts (http://127.0.0.1:3000/groups/flightjs/-/crm/contacts)
4. Go to an issue in a group project
5. Start typing /add_contacts [contact:
6. You should see some suggestions

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.

Related to #350135 (closed)