Let favourites and emoji reactions optionally be hidden

This MR allows hiding favourites and emoji reactions instance-wide by introducing a config setting, [:instance, :show_reactions]. The default is to continue displaying them as normal.

They are only hidden from the API. Of course it doesn't stop them from federating. Statistics are visible, but who favourited/reacted to the post is not.

The goal of this is to discourage outrage behavior. In my experience, almost nothing good comes from displaying favourites, and a lot of bad comes from it.

Soapbox FE already hides them by default, but this MR is to also prevent them from being displayed in third-party apps like Tusky, etc.