Rate-limit external requests

[thusoy]

We're flooding OMDb and eventually other resources now, we need to do some rate-limiting of requests to ease down outgoing requests.

[thusoy]

OMDb doesn't really care how much we flood them for now, since they don't use any sort of authentication, but we should still probably cool down a bit. If we later want to use TMDb for some stuff, like background images or awesomer cover images, we should impose a limit, but that's pretty easy with Celery. But we should probably be more restrictive about what triggers an external search as well, we're repeating requests quite frequently now...

Maybe store external searches done in a database, and only do the query if not already in the db, and then auto prune old entries?