Add speed limit attribute

[ampersand38]

When merged this pull request will:

• Add slider attribute to control vehicle speed limit
• Useful for matching vehicle speed to infantry, paradrop, heli search, etc
• https://www.youtube.com/watch?v=8V0OTt6pVrE
• Close #605

I messed up #648 and a new pr seemed like the easiest way to fix it. Attribute toggle settings split to #716