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