Add `Global Milestones` so that they can be assigned to projects when you create them

Moved from: https://dev.gitlab.org/gitlab/gitlabhq/issues/2474

Create a new milestone from the milestones view and choose which projects should be assigned this milestone.

Right now, if you want to have the same milestone across projects, so that it shows nicely grouped in the milestones view, you need to create them manually for each project. This can be annoying if you have to do it for more than 3 projects.