support: Pass various realm functions as template context.

We currently have code to calculate the value of realm_icon_url,
admin_emails and default_discount in two diffrent places. With
the addition of showing confirmation links it would become three.
The easiest way to deduplicate the code and make the view cleaner
is by doing the calculations in template. Alternatively one can
write a function that takes users, realms and confirmations as
arguments and sets the value of realm_icon_url, admin_emails and
default_discount appropriately in realm object according to the
type of the confirmation. But that seems more messy than passing
the functions directly to template approach.

zerver/tests/test_templates.py

@@ -154,6 +154,7 @@ of syntax errors.  There are two common causes for this test failing:
 
         """
         user_profile = self.example_user('hamlet')
+        realm = user_profile.realm
         email = user_profile.email
 
         context = dict(
@@ -194,6 +195,10 @@ of syntax errors.  There are two common causes for this test failing:
             invite_as={"MEMBER": 1},
             max_file_upload_size = 25,
             avatar_urls={"john@gmail.com": "www.zulip.com"},
+            realm_admin_emails=lambda _: "admin emails",
+            get_discount_for_realm=lambda _: 0,
+            realm_icon_url=lambda _: "url",
+            realm=realm,
         )
 
         context.update(kwargs)