Improve hosted cellxgene (#1234)

* Improve hosted cellxgene

 - option to turn off the test index page, or supply a page for redirect.
   For EB, The default is to return 404.  For cli launch, the default is the test page.

 - option to select which matrix types are allowed for multi dataset servers.
   For EB, The default is CXG only.  For cli launch, the default is any matrix type.

 - Return early with an error response if diffexp is requested when not configured

 - Verified that reembedings and user annotations also return with an error response
   if used when not enabled.

TODO:  The new options cannot currently be set by the user.
I plan to add a configuration file where these and all other settings can be set.

 Fixes #1210 
 Fixes #1228  
 Fixes #1229

server/eb/app.py

@@ -45,7 +45,9 @@ try:
         obs_names=None,
         var_names=None,
         anndata_backed=False,
-        disable_diffexp=False,
+        disable_diffexp=True,
+        multi_dataset_index=None,
+        multi_dataset_allowed_matrix_type=["cxg"],
     )
 
     matrix_data_cache_manager = MatrixDataCacheManager()