Support new Opencast series endpoint

[smarquard]

There is a performance regression in Opencast 3.x which affects XML deserialization (https://opencast.jira.com/browse/MH-12290). As a result, the /series/series.json endpoint is slower than in 2.x.

There is a new, faster endpoint in 3.x that only returns series title and ID: /series/allSeriesIdTitle.json

Galicaster should use this endpoint if available, as it is faster and produces lower CPU load on the server.