Fork suggestion becomes page-level modal

Context

Replace and Delete buttons from the file viewer are moved up to the header. When a user can't modify files directly in the project, we show the notification that offers to fork the project and do the change there instead. Previously when Replace/Delete was placed on the file viewer, that notification would show directly beneath. Now that we're moving it up, I feel the notification becomes a bit disconnected:

• placed far below the click target
• has default light grey background and doesn't stand out to help user notice it
• with expanded codeowners this may even be outside of view on certain viewport sizes

example of a new flow (don't mind disabled dropdown items)	example of a long codeowners list

Proposal

1. Introduce a page-level modal to display the contents of a current ForkSuggestion component.
2. Make a Delete file dropdown item a separate dropdown group, visually marked as destructive. Have Replace file and Delete file items trigger the fork suggestion alert.