Allow users to add an incident timeline event

What does this MR do and why?

Allow users to add an incident timeline event

Based on !81284 (closed), split into 3 MRs

Issue	MR
Display Incident Timeline Events	!89210 (merged)
Add timeline event manually	!89211 (merged)	<--- You are here
Delete timeline event	!89212 (merged)

Issue: #344059 (closed)

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

entire page	add timeline button	add timeline form

How to set up and validate locally

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

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.