Still had one instance of using the nonexistant supplier_name variable
that would cause a rendering error when a supplier had no
representatives, this fixes that error.

This adds a simple view (mostly cribbed from the component view but
without the database stats) to view details for a given supplier,
currently lists the components they supply and name/email address pairs for
representatives however there's room to add fields for other representative
contact and company information as well.

On the API side this adds 3 endpoints:
    * api/suppliers: returns a list of all suppliers and their
    representatives
    * api/suppliers/<id>: returns a supplier object containing the
    representatives for the given supplier
    * api/suppliers/<id>/components: returns a supplier object
    containing a list of components supplied by the given supplier

This also contains minor documentation and base template changes, fixing
the navigation menu to use class="active" on the currently selected page
(previously didn't work due to page_name capitalization)

This adds a basic Representative model and allows for editing them in
the admin interface both in the Suppliers admin panel for each
supplier and in a new Representatives admin panel which allows bulk
editing of all representatives. Currently multiple representatives are
just listed in the component view as extra rows below the company row
but it would probably make sense to add a view for viewing suppliers
directly.

The REST API has also been slightly modified to return a list of
representatives for each supplier in the components endpoint, adding a
suppliers endpoint would probably also be a good idea.

Requires running:
$ python manage.py migrate procurement 0002_add_representative

To update the database for the new model, both forward and lossy reverse
data migration is implemented in the migration.