Citadel: Compress very large access lists

People who are in hundreds of groups otherwise can't search for anything
because the request gets too large. Ideally Citadel accepted POST
requests, but those are already used for data modifications, so we are
stuck with GET for now.

citadel/indico_citadel/search.py

@@ -5,6 +5,9 @@
 # them and/or modify them under the terms of the MIT License;
 # see the LICENSE file for more details.
 
+import base64
+import zlib
+
 import requests
 from requests.exceptions import RequestException
 from werkzeug.urls import url_join
@@ -46,7 +49,12 @@ class CitadelProvider(IndicoSearchProvider):
                          **filter_query}
         # Filter by the objects that can be viewed by users/groups in the `access` argument
         if access:
-            search_params['access'] = ','.join(access)
+            access_string = ','.join(access)
+            if len(access_string) > 1024:
+                access_string_gz = base64.b64encode(zlib.compress(access_string.encode(), level=9))
+                search_params['access_gz'] = access_string_gz
+            else:
+                search_params['access'] = access_string
 
         try:
             resp = requests.get(self.records_url, params=search_params, headers=headers)