Set the default wagtail search backend explicitly

Resolves MS-544

server/core/settings.py

@@ -362,6 +362,11 @@ GRAPHENE = {
 
 # http://docs.wagtail.io/en/v2.1/advanced_topics/settings.html?highlight=urls
 WAGTAIL_SITE_NAME = 'skillbox'
+WAGTAILSEARCH_BACKENDS = {
+    'default': {
+        'BACKEND': 'wagtail.search.backends.database',
+    }
+}
 
 GRAPHQL_QUERIES_DIR = os.path.join(BASE_DIR, '..', 'client', 'src', 'graphql', 'gql', 'queries')
 GRAPHQL_MUTATIONS_DIR = os.path.join(GRAPHQL_QUERIES_DIR, '../mutations')