Rework how we bypass browser caches of the scouting app

This patch adds a new way to access files on the webserver. Users can
now request files by their sha256 checksum. The URL pattern follows
`/sha256/<checksum>/<path>`. The `<path>` portion doesn't actually
matter, but it helps with readability.

This patch migrates the main bundle and the pictures to use the new
checksum-addressable scheme.

Signed-off-by: Philipp Schrader <philipp.schrader@gmail.com>
Change-Id: I76eaa9b0f69af98e48e8e73e32c82ce2916fbe41
diff --git a/scouting/webserver/static/BUILD b/scouting/webserver/static/BUILD
index 3166853..0cf22f3 100644
--- a/scouting/webserver/static/BUILD
+++ b/scouting/webserver/static/BUILD
@@ -13,6 +13,7 @@
     name = "static_test",
     srcs = ["static_test.go"],
     data = [
+        "test_pages/index.html",
         "test_pages/page.txt",
         "test_pages/root.txt",
     ],