Pause menu

Changes:
- Added pause menu HTML and CSS
  - Added "Resume" button
  - Added "Restart" functionality
  - Pressing the "p" key opens the menu
  - "Settings" button currently does nothing
- Fixed HUD blinking bugs when going back scenarios or restarting
- Fixed camera selector not working for first and third person
- Camera switching now consistently hides the helicam overlays
- Fixed scenario 1 grid
- Modified startup order
  - Fixed bug with referencing hud.elements before hud is defined
  - Game starts rendering after intro scenario is completed

To do (future branches):
- Add functionality to "Settings" button
- Add "Switch Scenario" button and functionality

@kadst43 @AmbientOSX @ajiraffe