This adds the missing admin class, and registration of it, required to
actually change the value of the excluded extensions configuration.

A base URL can now be configured which is, potentially, prepended to an
asset URL.  This allows a CDN, or caching server, to front static asset
requests, taking load off of the contentstore and speeding up page load
times.

Asset URL generation respects locked vs unlocked assets, and will not
generate links to locked assets that would traverse a CDN (even though
the authorization component of the contentserver middleware wouldn't
allow those links to work anyways).