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
issue