Support seeding events

[derat]

As discussed here, it'd be nice to support seeding the "Add Event" form at https://musicbrainz.org/event/create.

Here's an example URL: https://test.musicbrainz.org/event/create?rels.0.target=e0195400-ad63-4a21-8d56-9a96647cb24e&rels.0.type=798&edit-event.name=Test&edit-event.type_id=1&edit-event.setlist=test&edit-event.period.begin_date.month=10&edit-event.period.begin_date.day=21

I haven't looked at this in any detail yet, but it looks like this form is seeded with GET instead of POST.