Add organization description field to realm settings.

This adds an organization description field to the Realm model, as well as
an input field to the organization settings template. Added three tests.
Set the max length of the field to 100 characters.

Fixes #3962.

zerver/lib/actions.py

@@ -431,6 +431,18 @@ def do_set_realm_name(realm, name):
     )
     send_event(event, active_user_ids(realm))
 
+def do_set_realm_description(realm, description):
+    # type: (Realm, Text) -> None
+    realm.description = description
+    realm.save(update_fields=['description'])
+    event = dict(
+        type='realm',
+        op='update',
+        property='description',
+        value=description,
+    )
+    send_event(event, active_user_ids(realm))
+
 def do_set_realm_restricted_to_domain(realm, restricted):
     # type: (Realm, bool) -> None
     realm.restricted_to_domain = restricted