Use consistent cache-control:max-age=600 for HTML pages

[nolanlawson]

Currently there is a cache-control:max-age=600 set for index.html, but no cache-control header set for the HTML pages served from routes. This PR makes both of them consistently use max-age:600 in production and no-cache in development.

In a future PR, I would like to make all of these cache-control headers configurable:

• immutable assets ('max-age=31536000, immutable' by default)
• routes, index.html ('max-age=600' by default)
• service worker ('max-age=0' by default, see Set service-worker max-age to 0 #428)

But for now I would settle for making them consistent. :)

[Rich-Harris]

Makes sense — thanks. More configurability would definitely be good in the longer term — I opened #415 the other day