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

To be able to more easily deal with same-origin issues, due to some course assets being loaded in an iframe, we're adding the ability to specify file extensions that should be excluded from URL canonicalization.  The default value is simply 'html', which means we won't rewrite asset links that point to files ending in .html.  Thus, they'll be loaded from the same origin as the parent page, and voila, no SOP issues.

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).