Make the speed unit a global, consistent setting

Right now the speed unit is configured independently in strafehud and physics panels, and is inconsistently calculated in the scoreboard if the cvar is changed in-game. This MR:

1. Makes the speed unit a global setting in the HUD settings menu (hud_speed_unit cvar)
2. Calculates the speed in the same unit consistently in all panels
3. Fixes the calculation factor in the scoreboard to show the correct value at all times

Fixes #2804 (closed)

The HUD settings menu is really full so I put it where I could, if there's a better place please tell me.