Added toast message to lock and notifications toggles

What does this MR do and why?

Adds a toast message when the request for toggling notifications and locking merge requests in the header is successful.

Screenshots or screen recordings

Screen_Recording_2022-06-10_at_11.08.50

How to set up and validate locally

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

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.

Related to #364574 (closed)