Move notification toggle back into sidebar

What does this MR do and why?

• Move the notification toggle back into the issuable sidebar
• Keeps the toast message when toggle is clicked

Why?

• Scrolling to the top to unsubscribe from an issuable was frequently cited as a frustration for users
• The visual indicator of knowing whether you are subscribed to an issuable at first glance was a valuable feature and was lost when the toggle moved into the dropdown
• We kept To do in a prominent, always-available location but until we have a solid notification center experience, we should keep both easily available

What are the trade offs?

• It has been placed back in its original location in the sidebar, meaning on smaller screens, it may not be visible (on issues in particular) without scrolling down
• It will no longer be available on the commits/pipelines/changes tabs in merge requests

Screenshots or screen recordings

	Before	After
Merge request
Issue
Epic
Incident

How to set up and validate locally

1 .Enable :moved_mr_sidebar 2. Check notification toggle on:

• Merge request
• Issue
• Epic
• Incident

UX Paper Cuts 16.2 → Settings UI (#398215 - closed)