Add guard condition to prevent intermittent KeyError

There are some circumstances where the context dictionary does not contain a
"request" item. In those cases we get a KeyError, for example:

  KeyError /clerk/api/email/{pk}/{email_id}/attachment/{attachment_id}/sharepoint/
  Level: Error
  Unhandled 'request'

Those circumstances are unclear however. It seems to be related to the issue
that was addressed in commit da95495 but I
cannot reproduce it. This change seems like a reasonable precaution until we
have more information.

app/web/templatetags/cookie.py

@@ -5,6 +5,7 @@
 
 @register.simple_tag(takes_context=True)
 def get_cookie(context, cookie_name):
-    request = context["request"]
-    result = request.COOKIES.get(cookie_name, '')
-    return result
+    request = context.get("request")
+    if request:
+        return request.COOKIES.get(cookie_name, "")
+    return ""