Close pop-up after successful fork operation

[ciyer]

Fix #723

Solution available for testing on https://sekhar.dev.renku.ch

To test -- fork a project and see that

1. The fork popup dialog disappears when the fork succeeds
   1.1 The dialog remains until the browser has loaded the new forked project
2. The dialog stays around when the fork fails and notifies the user of the problem.

Check on at least two browsers, as the behavior was different before on Chrome vs. Safari or Firefox.

[vfried]

Good fix, and good improvements to the code on top of the fix!