let the router handle dynamic files

the sb router already handles things like loading skin thumbnails and loading a file from the composer vendor folder through the ``load_thumbnail_from_skin()`` and ``load_file_from_vendor()`` functions, so I wonder why cant it also handle dynamic files (videos, profile pictures, thumbnails, etc)

PROS:
* hide private uploads
* hide uploads that require to be logged in from logged out users

CONS:
* adds overhead, would make the site a bit slower
* would fuck up with sb production instance's usage of nginx ratelimits.

Start by reading the SB router and its existing load_thumbnail_from_skin() and load_file_from_vendor() paths. Define which dynamic files require routing and authentication, then resolve the stated nginx ratelimit and performance tradeoffs before implementation; done criteria are not specified in the issue.

Written by the indexing model from the issue text.