Cache-busting was previously done with a compile-time timestamp. Change
to the SHA-256 hash of the file to avoid refreshing unchanged files.
The implementation could be used to add a new global fn (say,
get_file_hash) for subresource integrity use, but that's for another
commit.
Fixes#519.
Co-authored-by: Vincent Prouillet <balthek@gmail.com>
36ec33f042