linkbacks: fix tab visibility if excluded

* Adds module to excluded packages and disables tab in detailed record
  view.  (closes #1707)

* NOTE Adds deprecation warning for linkbacks module.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Looks good; two suggestions: let's point people to github directly, and let's express the "join us!" idea more explicitly:

* "The linkbacks module has been deprecated.  If you would like to use it, please join Invenio development on github."  (with a link)