context_processors: use a common context for emails.

Fixes #1611.

zerver/context_processors.py

@@ -4,12 +4,22 @@ from typing import Dict, Any
 from django.http import HttpRequest
 from django.conf import settings
 
-from zerver.models import get_realm_by_string_id
+from zerver.models import UserProfile, get_realm_by_string_id
 from zproject.backends import (password_auth_enabled, dev_auth_enabled,
                                google_auth_enabled, github_auth_enabled)
 from zerver.lib.utils import get_subdomain
 
 
+def common_context(user):
+    # type: (UserProfile) -> Dict[str, Any]
+    return {
+        'realm_uri': user.realm.uri,
+        'server_uri': settings.SERVER_URI,
+        'external_uri_scheme': settings.EXTERNAL_URI_SCHEME,
+        'external_host': settings.EXTERNAL_HOST,
+    }
+
+
 def add_settings(request):
     # type: (HttpRequest) -> Dict[str, Any]
     realm = None