Add character counter to incident timeline form

What does this MR do and why?

• Adds a character count to the incident timeline form.
• Prevents users from entering more than 280 characters.

issue: #372289 (closed)

Screenshots or screen recordings

before	after

How to set up and validate locally

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

1. Navigate to a project.
2. Navigate to the incident list: Monitor -> Incidents.
3. Create a new incident or open an existing one.
4. Navigate to the Timeline tab.
5. Select 'Add new timeline event' or edit an existing one.

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.