Fully merge milestones into versions view

[stefgers]

I don't like to much clicking on overview pages. So I would prefer:

• a single page called roadmap
• versions are visible and sorted by date (as in the current version menu item)
• milestones can be made visible in addition to the versions
• using a checkbox as for completed versions
• visibility of completed milestones can also be controlled via checkbox
• Milestones independant from any version (e.g. project management milestones collecting tasks) should be visible in the version view