User helper to hide inaccessible object page links.

Per [72359012](https://www.pivotaltracker.com/story/show/72359012).  Wraps the links to objects pages (`.viewLink`s) with the helper `is_allowed` so that they only render if the user would be able to access that page (i.e. read permission for `view_object_page` is true).

In cases where the anchor text must be rendered (and so the DOM element can't be hidden entirely), the `a` tag is conditionally replaced with a `span`.