Star plugin should store stars in local storage

[glaubinix]

If you star a talk, reload -> star is gone

This issue has probably a lot in common with the schedule change plugin - we need a way to identify the talk. Once we are able to do that this should be really easy.

[BattleRattle]

But I think, there's no way of identifying a talk, since every detail (title, speaker, room, description) might change. The data source itself has to provide such an identifier.