Agg evicting queue

Deprecates #1282 and solves #1214 and #1216.

Behaviour is as follows:
 * Users are free to download
 * If an IP address is "abusing" us with too many aggregator downloads, it gets penalized with a captcha
 * `trustedClients` in configuration can always download without a captcha
 * `maxAggregatedDownloadsInTenMinutes` set to 2 by default (configurable)
The value for the id attribute should be quoted `html: "<div id='challenge'></div>"`