Allow overriding SEARCH_FILTER_GENERATOR

This allows setting a path to a custom python module/plugin for search.
For example, if one wishes to filter out certain results.

lms/envs/production.py

@@ -805,6 +805,7 @@ if FEATURES.get('ENABLE_COURSEWARE_SEARCH') or \
    FEATURES.get('ENABLE_TEAMS'):
     # Use ElasticSearch as the search engine herein
     SEARCH_ENGINE = "search.elastic.ElasticSearchEngine"
+    SEARCH_FILTER_GENERATOR = ENV_TOKENS.get('SEARCH_FILTER_GENERATOR', SEARCH_FILTER_GENERATOR)
 
 ELASTIC_SEARCH_CONFIG = ENV_TOKENS.get('ELASTIC_SEARCH_CONFIG', [{}])