[Form] port prestacms url creation widget to cmf

[dbu]

It would be great to have the prestacms URL selection widget as a form type available in the cmf. having the logic for this in the backend and update the proposal on updating the title field would be great. then we could leverage the RoutingAutoBundle configuration to generate proposals.

http://sandbox.prestacms.com/admin/en/cms/page/edit?locale=en&website=%2Fwebsite%2Fsandbox&id=website%2Fsandbox%2Fmenu%2Fmain%2Fprojects

/cc @nicolas-bastien @dantleech