draft: allow sharing pages between questions

• you can put Q1 and Q2 on the same page
  • or make Q1a and Q1b independently markable
• Fix some places where the code assumed there is just one FixedPage for a given page number and test number
  • there may be other places
• in theory, you could even have multiple versions on the same page BUT our "versionN.pdf workflow" prevents this from happening.
• Future work: re-enable the spec-checker errors as warnings, b/c folks probably don't want this feature on their first rodeo!
• client displays a warning when you select a question that shares a page
  • TODO: this refers to docs that don't exist yet.
• temporarily changed the default spec to play with this:
  • change back before merging
  • or learn how that config stuff works and try to add this demo.