Add a way to disable a backend alltogether.

Fixes fedora-infra/fmn.consumer#3.

fmn/lib/__init__.py

@@ -90,10 +90,14 @@ def matches(filter, message, valid_paths, config):
 def load_preferences(session, config, valid_paths):
     """ Every rule for every filter for every context for every user.
 
+    Any preferences in the DB that are for contexts that are disabled in the
+    config are omitted here.
+
     This is an expensive query that loads, practically, the whole database.
     """
     preferences = session.query(fmn.lib.models.Preference).all()
-    return [preference.__json__() for preference in preferences]
+    return [preference.__json__() for preference in preferences
+            if preference.context.name in config['fmn.backends']]
 
 
 def load_rules(root='fmn.rules'):

fmn/lib/tests/__init__.py

@@ -11,7 +11,9 @@ def setUp(self):
             os.unlink(dbfile)
         self.sess = fmn.lib.models.init(DB_PATH, debug=False, create=True)
 
-        self.config = {}
+        self.config = {
+            'fmn.backends': ['irc', 'email', 'android'],
+        }
         self.valid_paths = fmn.lib.load_rules(
             root='fmn.lib.tests.example_rules')