Video downloads through the central server

See issue #614

Seems worthwhile to again move to a general strategy of pointing things to our central server first, allowing us to intervene anytime we want.  Specifically, this allows us to:
* Switch out video encodings 
* Change download locations
* Handle back-end errors gracefully
* Collect stats

The server cost should be minimal--redirects are lightweight, and could always be served by a frontend (apache/nginx) if we wanted to.  But the flexibility is immense.

Testing:
* I tested that video downloads work
* I tested that central server links (wiki, "about") work
* I did not test any codepath using /image/* (used to be knowledge map, but we stopped doing that for efficiency reasons)