fundgoal: Add provider convenience methods to FundraisingGoal.

www/conservancy/apps/fundgoal/models.py

@@ -1,3 +1,5 @@
+import random
+
 from django.db import models
 
 class FundraisingGoal(models.Model):
@@ -18,6 +20,18 @@ class FundraisingGoal(models.Model):
     class Meta:
         ordering = ('fundraiser_code_name',)
 
+    def providers(self):
+        return GoalProvider.objects.filter(fundraising_goal=self)
+
+    def random_providers(self, k=None):
+        providers = self.providers()
+        if not providers.exists():
+            return None
+        elif k is None:
+            return random.choice(providers)
+        else:
+            return random.sample(providers, k)
+
 
 class GoalProvider(models.Model):
     fundraising_goal = models.ForeignKey(