Support packages occurring multiple times

[Jumpy-Squirrel]

Package configuration should include a maximum count that defaults to 1.

If a higher maximum count is configured for a package, allow selecting it multiple times.

All resources that currently take a comma separated list for packages should support receiving a package multiple times in the list.

We may also add a new additional field that can be used instead, and that takes a map of packages, such as

packages: {
  tshirt: {
    count: 2
  }
}

or something similar. When reading an attendee, both fields should be filled, when writing, the new field takes precedence.

The first step of the implementation should be a proposal for how the API spec would be expanded by this. Please do not start implementation before this has been discussed and reviewed by me.

[Jumpy-Squirrel]

Usability enhancement: optionally allow specifying an explicit list of allowed counts in configuration, and check this in the backend.

[Jumpy-Squirrel]

The GET packages/{package} endpoint should return the count, in addition to the "present" boolean.