Invaders Face (#210)

* invaders face: Initial commit, fully functional so far

* invaders face: silence compiler warning

* invaders face: prevent involuntary restarts when the game is over and save some bytes on flags

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Co-authored-by: joeycastillo <joeycastillo@utexas.edu>

movement/movement_faces.h

@@ -80,6 +80,7 @@
 #include "habit_face.h"
 #include "repetition_minute_face.h"
 #include "timer_face.h"
+#include "invaders_face.h"
 // New includes go above this line.
 
 #endif // MOVEMENT_FACES_H_