Should we be able to unboost an object after boosting it ?

# Context
I'm working on displaying the number of likes, replies and boosts on objects in mastopod and realized that as much as we can currently boost `Notes` we can't "unboost" them.

I've implemented the `shares`collection on object in semapps : https://github.com/assemblee-virtuelle/semapps/pull/1369 but I religiously sticked to the [AP spec](https://www.w3.org/TR/activitypub/#shares) that only states that objects MAY have a `shares` collection containing a list of announces targeting it.

# Proposition
Emulate the way [`likes` are handled](https://www.w3.org/TR/activitypub/#likes) by adding a `shared` collection on the actor containing the objects he has shared.

This way the `boost` button could be toggled to `unboost` on already boosted objects.

Any thoughts on this @srosset81 @Laurin-W ?