Fork button on Blob#show disregards already forked projects

It appears we have a new feature, where if I click 'edit' while I don't have the appropriate access GitLab will show a green button to fork.

However, if one already did this in the past, we show a blue banner with 'A fork has been created '. This should be improved to:

1. detect if we already forked a project
2. Redirect after a click