ReportbackFile Entity

* Adds ReportbackFileEntity class to facilitate issues like #3564 and #3597

* Fixes access callback for `admin/users/rb`
Ehh, I mean.. i'm ok to leave it :) I'm specifically passing `NULL` as the argument for the `$entity` in `dosomething_reportback_menu`. and then Nodes and Taxonomy Terms for the other callbacks where this function is called.