Add Edit Timeline Event Functionality

What does this MR do and why?

Describe in detail what your merge request does and why.

Adds the functionality to edit timeline event.

This MR is the final part of a series of 5 MRs, all other MR's must be merged before this one is merged but it is ready for review now:

graph TD;
A[Add Edit Timeline Event Functionality] --> B[Refactor Timeline Event Form];
A[Add Edit Timeline Event Functionality] --> C[Refactor timeline event list];
A[Add Edit Timeline Event Functionality] --> D[Refactor timeline events list tests];
B[Refactor Timeline Event Form] --> E[Split Create timeline event from Timeline Event form];

The links are:

• Add Edit Timeline Event Functionality !95061 (merged) (you are here)
• Refactor Timeline Event Form !94899 (merged)
• Refactor timeline event list !95073 (merged)
• Refactor timeline events list tests !95046 (merged)
• Split Create timeline event from Timeline Event form !93545 (merged)

Screenshots or screen recordings

How to set up and validate locally

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

1. Within a project on the left hand menu bar navigate to Monitor > Incidents
2. Open an incident (create one if needed)
3. Using the tab headings, select Timeline
4. Create a timeline entry if there isnt one
5. On the right hand side of the timeline event select edit from the more options symbol

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 #344060 (closed)