An app's page says how long its golden path took: New app to the first preview, and to production #3668

For building apps fast: the owner asked for apps to be built super fast, and the forge cannot show whether they are. The app record has createdAt, and deploy runs have servingAt, so the figure can be computed. It is not shown anywhere.

Done:

• [ ] One plain function, (db, app) in, { firstPreviewMs?, firstProductionChangeMs? } out. It returns the time from the app's creation to the first preview environment that served, and to the first production deploy after the first merged pull.
• [ ] The app's page shows it as one dense line, for example "first preview 2m 14s after New app · in production 9m". The line is left out while there is nothing to show.
• [ ] The same figures are in the app's JSON from GET /api/v1/repos/{owner}/{name}/app.
• [ ] Tests cover an app with no preview yet, with a preview, and with a merged change.

Files: a new module in src/apps/ named for the figure, src/web/pages/apps.ts, src/http/api.ts, a test. Fits: a page builder.