[admin] add support for re-encoding video Previews about madek

[eins78]

@michalpodlecki Idea: just send the first batch? (100, or maybe a better value than can be found by looking at their docs again)?

We already assume that we won't be able to send all missing previews in one go because
a) the controller/proxy/browser will time out at some point
b) we send to fast and hit the rate limit, so end of the batch will result in failures.

This would also solve the problem of "concurrent batches", the button is clicked and there will a response in reasonable time.
Everything would cleanly be persisted to the DB, and the button could be immediately clicked again.
In this szenario it wouldn't even be too hard to really check for the rate limit - we can just query zencoder_jobs to find out how much we've sent.

[eins78]

@michalpodlecki the JS client is working with a mock controller, you've already seen it ;)
Madek/madek_admin-webapp@35d4fa3

[eins78]

@michalpodlecki update: Madek/madek_admin-webapp@Madek:bd19297...Madek:2775e93

[eins78]

see #20