fix reshares in single post-view

Fix for #4023

The behaviour of the reshares after this pull request is the following:

* A user can reshare a post from the stream and from the single post view (even if it was reshared by somebody else).

* In the stream, if you try to reshare a post that you already reshared you get the following flash message: "That good, huh?  You've already reshared that post!"

* If you open in one tab a post and reshare that post in the stream. If you try to reshare that post in the tab that shows the single post-view -without hitting refresh- you get the flash message: "That good, huh?  You've already reshared that post!"
And it works the other way around.

A review is welcome!