Update it2 Course

 * Improve balancing of rare words (less blocks repetitions)
 * Remove vulgar words
 * Make different lessons have different words (in symbols and numbers
   lessons)
 * Make lesson about 1000 characters long (learning seems less tiresome)
 * Add lesson with all the characters
 * Add final lesson with all the characters and longer text to train
   endurance

Contributed by Simone Gaiarin

Differential Revision: 18902