Add a sharing link to collections

This commit introduces a "sharing link" to the decklist collections, allowing the organizer to share the list of players without having to look at them by themselves, for instance to share it with a caster crew or with a judge, without them needing to have a login on unityleague. Of course, we cannot prevent the organizer themselves from looking, but at least it requires an additional click.

This is implemented with a "sharing key", 128 bit passwords that can be added to the GET request for the Collection as a way to "unlock" the access to the links. Doing it like that instead of through a primary key keeps the URL stable and avoids introducing another key that would need to be indexed for instance.