Convert "The merge request can now be merged" alert to a toast

Following up from #338699 (comment 670078555)

Let's convert this alert to a toast

Proposal

• Convert existing alert (shows up when you remove draft status) into a toast
• Add a toast when user marks as draft

Text

When marking as draft:

Marked as draft. Can only be merged when marked as ready.

When marking as ready:

Marked as ready. Merging is now allowed.