Federation issue: "announcement" should not be required to create events from federation.
Federation issue: Announce with nested objects breaks federated listing of events. When Mobilizon receives an Announce from an instances application actor it follows it adds the `id` of the `object` within the `Announce` to the shares table. But the specs don't enforce that the actual content is announced directly. So for example if this announce is sent: ```json { "type": "Announce", "id": "some announce id", "object": { "type": "Create", "id": "some creation id", "object": { "type": "Event", "id": "Some event id", } } ``` the event with the id `some event id` is added to the `events` table but in the `shares` table the the `id` of the `Create` "some creation id" is added in the `uri` column. Because of that this event is not listed on the start page and search results. Linked to #1391
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