Resolve "Add Incident Tag to the 'edit item' form"

What does this MR do and why?

• allows to edit the tags, when timeline event is edited
• formats retrieved event tags into an array
• documents timeline event tags feature

Screenshots or screen recordings

before	after

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Open the Rails Console in a terminal under the gitlab/ folder: rails c
2. Run Feature.enable(:incident_event_tags) to enable to feature globally for your GitLab instance.
3. Navigate to Monitor > Incidents page
4. Create new or open an existing incident
5. Navigate to Timeline tab of the incident
6. Click "Edit" button in the More dropdown of a specific event
7. Choose a tag from dropdown
8. Save the changes

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 #373854 (closed)