Possible optimizations of the activity feed

* [ ] In some cases the feed might return 2 cards for the same outing and the same user. For instance:

https://www.camptocamp.org/outings/853608/fr/wittenberghorn-witteberghore-par-le-plateau-de-ober-meiel-depuis-grund

Here is what has probably happened according to @tsauerwein

> * User bibotte creates the outing and adds BertrandSemelet as participant
> * BertrandSemelet uploads images to the outing. This creates the first feed entry.
> * Then BertrandSemelet adds other participants to the outing. This is handled by update_participants_of_outing. This function takes the feed entry by bibotte, and changes the user from bibotte to BertrandSemelet. Now, there are two feed entries for BertrandSemelet.
>
> The way update_participants_of_outing selects an existing feed entry and changes the user is probably not optimal.