Hook up event series and events in the sidebar

Building on the work in https://github.com/CUL-DigitalServices/grasshopper-ui/pull/37 the event series and events should be fetched for courses and parts selected in the pickers.

The back-end work for this hasn't been merged yet so the https://github.com/fronteerio/grasshopper/tree/groups-events-series branch should be used for development.
Same here, the first check can be dropped. Might also want to do this in all the other validation.