Change 'related materials' field, fixes #5780

Add autocomplete field to link to related image descriptions

Squashed commit of the following:

commit 070534c7a15baa0c4c4fc5c6260297b2901b12bb
Author: José Raddaoui Marín <raddaouimarin@gmail.com>
Date:   Mon Oct 21 20:13:58 2013 +0200

    Use fixed id for the new relation type term

commit a4d7adc4ed8549f5367a338644fe364bc5823875
Author: José Raddaoui Marín <raddaouimarin@gmail.com>
Date:   Sun Oct 20 20:50:50 2013 +0200

    Show 'Related material descriptions' field

    Add partial to show linked archival descriptions as related materials
    in the index page (ISAD and RAD templates)

commit 1a00c3b8f40480cce550e3ac14fb6efa99f9ea06
Author: José Raddaoui Marín <raddaouimarin@gmail.com>
Date:   Sun Oct 20 20:47:07 2013 +0200

    Add 'Related material descriptions' field

    Add autocomplete field to allow linking multiples archival descriptions
    as related materials to the resource in the ISAD and RAD templates

commit 7890d0ef13007c8f9e26555a9ad6ef9fcfca087e
Author: José Raddaoui Marín <raddaouimarin@gmail.com>
Date:   Sun Oct 20 18:24:28 2013 +0200

    Add 'Related material descriptions' term