Allow importing rides

[navaronbracke]

There was a feature request for the option to import attendees for a specific ride.
We should support that.

We currently have a way to sync riders, by importing them from a file.
But we do not have a way to import rides (their date + attending information), which is what this would be adding.

TODO:

• Add a button to 'import' rides (similar to the import riders button)
  • This button opens the import ride screen
• This screen allows selecting a JSON/CSV of a single ride or an entire ride calendar (see the export format)
  • Parses the content of the file into a data structure that contains the dates + identifiers of the attendees (first name, last name, alias)
• Loads the list of riders, so that the incoming attendees can be verified
  • Keep track of the persons that are not yet recognised, so that this can be displayed later as an informational view
  • At the end of the import, show a list of people that were not recognised, with a hint to import them using the correct page (add a navigation button?) This can be a list view, as the ride dates are not relevant here.
• For each ride
  • if the ride does not exist, add it first
  • add all the attendees to the ride that don't exist for it yet (skip existing attendees, and if the end result is no additions, skip updating the ride entirely)
• Refresh the list of rides + the list of attendees (both might have changed)

TBD:

• How are we going to optimize multiple rides at once?
  • Do we need a confirmation page to show what will be added?