Consolidate information from data.edx.org, Appendix E, and problem types

@mhoeber @lamagnifica This PR is part of the giant problem reorg project. Some things still don't look good (I haven't gone over the writing for the "Conditional Modules" section, for example)--this is an intermediate step. The next steps will be to break each problem type into its own page and do a good scrub for each one. (In other words, don't worry so much about nits for this review--just look for egregious things.)