tour: improve local storage handling of files

this supports adding new pages or removing them without invalidating
local caches of files that didn't change

Fixes #13252

Change-Id: Ibcc400347551de7f5ed1e58250a9c1cbb437cbaa
Reviewed-on: https://go-review.googlesource.com/17407
Reviewed-by: Chris Broadfoot <cbro@golang.org>
3 files changed