Cannot boost someone else post

If I try to boost someone else's post, I get this error:

> Only the creator has the right to share the object

It works if I boost my own posts.

Indeed, `Announce` activities are currently handled by the AnnouncerService, which is considering "announcing" as a way to give view permissions to an object, and thus restrict it to the owner.

Instead of throwing an error, we could simply ignore the permissions part if the announcer is not the creator of the object.

But this is more difficult when receiving a `Announce` activity, because the AnnouncerService store it in cache.

In the future, the idea will be to use capabilities to share resources, so the AnnouncerService will disappear.

But is there something quick to do in the meanwhile ?