Column config UI, and template fixes

[mccalluc]

A lot going on here:

• fix Prompt for each numeric column #33 / fix Per column preview graph #83 / fix Per column code preview #84
  • Shiny code gets a lot more complicated: parameterized module, passing event handlers, etc.
  • DP preview! Not 100% confident about the details, but you can slide epsilon up and down and it does what you'd expect.
• Templates:
  • Reorganize template code: We used to have template.py and a templates/ directory. Consolidate: The python file is now the __init__.py in the directory, and the templates themselves have been moved down a level, inside no-test/.
  • Make the template filling code more robust:
    • Check for word boundaries when filling slots
    • Remember the slots that need filling, and only warn if those are unfilled at the end (because the user supplied strings may be all caps)
• A couple more code examples in the UI, where they make sense
  • Code samples now have titles
• Simpler code for plotting inside the column config block.
• Fix the math for cutpoints on bins.
• Add doctests
• debugging helpers
  • run tests and coverage with ci.sh
  • Save playwright artifacts on failure (so we can see what went wrong on github, if it works locally)

Not in the PR:

• UI is ugly: This is far from matching the mockups.
• Not super confident that the DP is being done the best way. (See noisy counts vs. counts of noise? #100)
• UI tests could be stronger. Maybe kill two birds: What if we included a small table of results with every graph, and this text would be easy for Playwright to target? (For example, for the confidence interval.)