634 fork and unmirror repository

Shall fix both issues of #634 (and #351): Allowing to fork a repository and converting a mirror to a local repository.

This also improves the javascript in the new repository view as it now updates everytime the source address is changed (key press, click, cut, paste) and not only when its input field loses focus.
Style/NegatedIf: Favor `unless` over `if` for negative conditions.