Improve nested versions

The usability of nested versions could be improved. Currently:

• You need GreenHopper.
• Nesting the versions occurs inside GreenHopper and not in the Managing Versions page where most naive users would look.
• You need to enable a project to be GreenHopper enabled. If you have hundreds, this gets daunting
• It pollutes the fixversion field with multiple fixversion values. This isn't a big deal but it's a consequence of the workaround GH had to do.
• Delegating this task to project admins isn't always possible due to the learning curve. Here's a doc I found on how one might do it.