Incident management timeline event's preview does not work

Timeline event's preview does not render the content

Steps to reproduce

1. Open an incident
2. Navigate to "Timeline" tab
3. Click "Add new timeline event" button
4. Provide any text for the event
5. Click on the Preview tab

Demo

TimelineEventPreview

Implementation details

• backend Add a preview endpoint for timeline events
• frontend Re-enable Preview button and make it use the preview endpoint