set up route placeholders, /cases, /casesummary

[jkoren]

This PR builds on the routes PR 125.

• The case summary now displays with a /casesummary route and not on every page.
• The default route / goes to a placeholder home page for the app, which for the moment has a list of routes.
• The /cases route still goes to a placeholder list of cases

A new /routes/_heat+ directory was created to hold .tsx files for all new routes created for the HEAT calculator. The default index.tsx page was also moved here.

[thadk]

With git push origin setting-up-route-placeholders:dev, particularly pushing to the special dev branch (welcome to force push if you need to), I deployed this to https://heat-stack-staging.fly.dev/ for checking. LGTM so far.