Progress Indication for Resync

[KatrinIhler]

When initiating a complete resync, we currently get only the date in the log with which the harvest API is called in that moment, so we know Tobira is asking Opencast for all elements updated after that date. But it's hard to figure out how far along the resync is based on that. Something like "x of y events resynced" or even "z events left to sync" would be more helpful.

[LukasKalbertodt]

To show that information, the harvest API need to provide it. In particular, the harvest API would need to tell us the total number of events, series and playlists that would be sent if preferredAmount would be set to ∞. The search service (from which we receive events) very easily provides that number for us. The series and playlist service do not, so we would have to adjust the getForAdministrativeRead method. I haven't dug deeper how difficult it would be to do that. Should be fairly straight forward?

Sure, events are the most important metric, but it feels kind of dirty to just expose this information about events.

But yeah, someone would have to look a bit deeper into that.