Report project/group abuse when leaving

In https://gitlab.com/gitlab-org/gitlab-ce/issues/59421, we cut scope significantly because of required backend and UX.

Users cannot yet report the project/group using the abuse reports feature after/when they leave the project/group.

We would need UX to decide how to approach this. Ideas so far...

1. Show a report link/button in the successful "you have left the project/group" flash notice.
2. Instead of redirecting to dashboard, redirect to some page where there is a message and a report button.
3. Toast message on success with a link to report