Allow suppliers to have multiple representatives

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.

1 files changed, 9 insertions, 3 deletions

diff --git a/procurement/models.py b/procurement/models.py
index 1e70736..43e5a07 100644
--- a/procurement/models.py
+++ b/procurement/models.py
@@ -51,19 +51,25 @@ class DashboardModel(models.Model):
         base_string = 'updated {quantity:.2f} {units} ago'
         return base_string.format(quantity=quantity, units=units)
 
-
 class Supplier(DashboardModel):
     """
     Model which represents an individual or organisation which supplies components
     """
     name = models.CharField(max_length=255)
-    representative_name = models.CharField(max_length=255, null=True, blank=True)
-    representative_email = models.EmailField(max_length=255, null=True, blank=True)
     is_authorized = models.BooleanField()
 
     def __str__(self):
         return '{}'.format(self.name)
 
+class Representative(DashboardModel):
+    """Model which represents a single Representative, each supplier
+    can have multiple representatives"""
+    name = models.CharField(max_length=255)
+    email = models.CharField(max_length=255)
+    supplier = models.ForeignKey(Supplier, on_delete=models.CASCADE, related_name="representatives", null=True, blank=True)
+
+    def __str__(self):
+        return '{} <{}>'.format(self.name, self.email)
 
 class Component(DashboardModel):
     """