fix reshares in single post-view

[fabianrbz]

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!

[jhass]

Looks good to me :)

[fabianrbz]

Changelog updated!

[Raven24]

thank you very much!

[jhass]

Great, thanks! Could you ammend the changelog update to the original commit in the future? This way we won't get that many meaningless "update changelog" commits in the history :)

[fabianrbz]

sure!