More systems questions and lab step progress

What changed

• Build a System gained about 30 new questions, each modeled on an interview format a company has publicly been reported to use. Each one was checked by running a correct solution and deliberately broken ones against its tests.
• Course labs now show how far along you are, such as 3 of 8 steps done, with the next step named. Rena marks steps from your prompts in Builder labs and from the end-of-lab review in every lab.
• Your profile's streak now counts days in a row you were active, and your solved totals count each problem once. Educators who joined a company or switched to the student view can still create assignments and host live games.