Load new manifest in a clean page

[bhollis]

The manifest has gotten too big! Trying to reload it in place is failing on phones because old manifest + new manifest = browser crash. Instead we should redirect to a page that does nothing but download the manifest into IDB, and then reload back into the main DIM App. Not as nice as just updating in place, but more memory efficient.

Another complementary option is to start hosting a proxied version of the manifests that's stripped down (useless crap removed)...