add app_label to models: fix breaking tests

tests in Open edX `common` don't have the `organizations` in the
INSTALLED_APPS this change and throws an error like:

"Model xyz is not in INSTALLED_APP and doesn't declare `app_label`"

This fixes it by adding app_label.

Also, removing unneeded @python_2_unicode_compatible now we're past
Python 2.

organizations/models.py

@@ -48,6 +48,9 @@ class Organization(TimeStampedModel):
 
     history = HistoricalRecords()
 
+    class Meta:
+        app_label = 'organizations'
+
     def __str__(self):
         return u"{name} ({short_name})".format(name=self.name, short_name=self.short_name)
 
@@ -76,7 +79,6 @@ def clean(self):
                                     'and underscore (_).'))
 
 
-@python_2_unicode_compatible
 class OrganizationCourse(TimeStampedModel):
     """
     An OrganizationCourse represents the link between an Organization and a
@@ -93,6 +95,7 @@ class Meta:
         unique_together = (('course_id', 'organization'),)
         verbose_name = _('Link Course')
         verbose_name_plural = _('Link Courses')
+        app_label = 'organizations'
 
     def __str__(self):
         org = self.organization
@@ -102,7 +105,6 @@ def __str__(self):
         )
 
 
-@python_2_unicode_compatible
 class UserOrganizationMapping(models.Model):
     """
     User membership in an organization.
@@ -115,6 +117,9 @@ class UserOrganizationMapping(models.Model):
     is_active = models.BooleanField(default=True)
     is_amc_admin = models.BooleanField(default=False)
 
+    class Meta:
+        app_label = 'organizations'
+
     def __str__(self):
         org = self.organization
         return 'UserOrganizationMapping<{email}, {organization}>'.format(