Add timeline event manually

Proposal

Create a timeline event from manually in the UI.

Design

Add new event button	New event form

• Introduce a button for adding a new timeline event
• Button triggers a form with fields for adding timestamp and text content
• Note: people can only input a maximum of 280 characters in the timeline text field. So, the editing field should utilize the character counting field variant from Pajamas, as outlined in these docs.

Figma

Frontend

• Show form for creating event
• Call API when submitted

Backend

• Provide API to add a timeline event

See Figma for latest designs: https://www.figma.com/file/01efUzilO8y3FZndx4Kt14/Incident-timelines-gitlab-com-and-330901?node-id=63%3A342