Draft: Remove the `:incident_timeline` feature flag
What does this MR do and why?
Enables the incident timelines feature.
Also incorporates some frontend changes, based on pre-release UX feedback.
Important note: Feature flag rollout is still in progress, this MR is being prepared ahead of time so it's ready to go.
See the rollout issue.
Screenshots or screen recordings
| incident timeline tabs | 
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| Incident timeline | 
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| Incident timeline form | 
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How to set up and validate locally
- Navigate to a project.
- Navigate to the incident list.
- Monitor -> Incidents in the left menu.
 
- Create a new incident.
- Navigate to the Timeline tab.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR. 
Edited  by Tristan Read