remove `shared_url` helper (bug 1024185)

fixes [bug 1024185](https://bugzilla.mozilla.org/show_bug.cgi?id=1024185)
yes indeed, and at the moment, none of this PR works properly from what I can tell (`get_reviews_url` seems to be for personas/themes, as it's using `rating.themes`, but it's used for addons, and `addon_url` doesn't exist).

I need to rework this PR, it's not ready yet, but I forgot to add "WIP" in the name of the PR.