Added toast message to widget rebase action

What does this MR do and why?

Adds a toast message to the merge request widget rebase state.

Screenshots or screen recordings

Screen_Recording_2022-06-21_at_10.51.51

How to set up and validate locally

1. Set a project's merge method to Merge commit with semi-linear history
2. Create a new merge request with change
3. Update the main branch and the Rebase button should appear on the merge request created

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.

Closes #299907 (closed)