Improve user experience around quitting & moving between tutorial challenges

[byorgey]

• Help modal now says "quit the current scenario" instead of "quit game" for Ctrl-q
• If you select "Keep playing" after completing a challenge scenario, it will now pop up a modal advising you to hit Ctrl-q whenever you're ready to move on.
• Added a note at the beginning of the tutorial reassuring that your progress will be saved and you can pick up where you left off from the menu.
• Improved quit dialog in a couple ways:
  • Now warns that "your progress on the current scenario will be lost" (to emphasize that e.g. your progress on the entire tutorial won't be lost).
  • Now says "quit to XXX menu" where XXX is the specific menu you will return to.

Closes #595. Closes #759.