Improve project fork button

Proposal

The current fork button on a project has a quite "intricate" behaviour, because it serves multiple purposes:

• it links to the user's fork if, and only if, the user has forked it and has access to only one writable namespace, namely only if the user does not have write access to any group
• otherwise it shows a link to the new fork page

This makes it hard to quickly go to a fork of the project that the current user has actually access to. The only way to do so would be to go to the project forks page (using the forks count button near the fork button) and search for a fork the user has access, which can be basically impossible (for example the GitLab project has more that 8000 forks, and there's not filter for access level on the forks page).

What I am proposing is to change how that button works:

• Change the fork button from a link to a dropdown
• Remove the forks count button and incorporate the number as a badge inside the dropdown button text
• The dropdown will show the list of forks the current user has write access to
• On the dropdown there will always be a button to create a new fork
  • If the user cannot create a fork, the button will be disabled and a warning will be shown with the reason why
• On the dropdown there will always be a button/link to the forks page (replacing the current forks count button)