Resolve "Use toggle for subscribed/unsubscribed to notifications"

Turns the issuable sidebar "Subscribe"/"Unsubscribe" button into a toggle switch, and changes the collapsed icon from an RSS icon into a bell (or a bell with a slash if notifications are disabled).

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #16988 (closed)