templates: Add new context variables to all templates.

This adds a few new helpful context variables that we can use to
compute URLs in all of our templates:
* external_uri_scheme: http(s)://
* server_uri: The base URL for the server's canonical name
* realm_uri: The base URL for the user's realm

This is preparatory work for making realm_uri != server_uri when we
add support for subdomains.

zerver/worker/queue_processors.py

@@ -158,6 +158,9 @@ class ConfirmationEmailWorker(QueueProcessingWorker):
                                               'referrer': referrer,
                                               'verbose_support_offers': settings.VERBOSE_SUPPORT_OFFERS,
                                               'external_host': settings.EXTERNAL_HOST,
+                                              'external_uri_scheme': settings.EXTERNAL_URI_SCHEME,
+                                              'server_uri': settings.SERVER_URI,
+                                              'realm_uri': referrer.realm.uri,
                                               'support_email': settings.ZULIP_ADMINISTRATOR},
                                              datetime.timedelta(days=2),
                                              tags=["invitation-reminders"],