realm: Add registry of realm property types.

This makes it possible for us to do some convenient validation for
developers, checking whether the correct types are passed for each
each realm property.

zerver/lib/actions.py

@@ -422,9 +422,11 @@ def active_humans_in_realm(realm):
 
 
 def do_set_realm_property(realm, name, value):
-    # type: (Realm, Text, Union[Text, bool, int]) -> None
+    # type: (Realm, str, Union[Text, bool, int]) -> None
     """Takes in a realm object, the name of an attribute to update, and the value to update.
     """
+    property_type = Realm.property_types[name]
+    assert isinstance(value, property_type)
     setattr(realm, name, value)
     realm.save(update_fields=[name])
     event = dict(

zerver/models.py

@@ -144,6 +144,23 @@ class Realm(ModelReprMixin, models.Model):
                                       default=2**31 - 1) # type: BitHandler
     waiting_period_threshold = models.PositiveIntegerField(default=0) # type: int
 
+    # Define the types of the various automatically managed properties
+    property_types = dict(
+        add_emoji_by_admins_only=bool,
+        create_stream_by_admins_only=bool,
+        default_language=Text,
+        description=Text,
+        email_changes_disabled=bool,
+        invite_required=bool,
+        invite_by_admins_only=bool,
+        inline_image_preview=bool,
+        inline_url_embed_preview=bool,
+        name=Text,
+        name_changes_disabled=bool,
+        restricted_to_domain=bool,
+        waiting_period_threshold=int,
+    )
+
     ICON_FROM_GRAVATAR = u'G'
     ICON_UPLOADED = u'U'
     ICON_SOURCES = (