diffexp limit UI and configuration (#1336)

* warning on maxCount for diffexp

* cleanup logging

* clarification

* make the limits configurable

* make diff exp limit work

* danger!

* remove debugging code

* fix merge with master

* fix unit tests

Co-authored-by: Colin Megill <colinmegill@gmail.com>

server/common/default_config.py

@@ -80,6 +80,10 @@ adaptor:
   anndata_adaptor:
       backed: false
 
+limits:
+  column_request_max: 32
+  diffexp_cellcount_max: null
+
 """