Start refactoring of the trigger system:

- Move each trigger handler into its own file
- Add dictionary helper classes for easier reading and writing of dict-based data
- Extract the web hook payload -> internal representation building for each trigger system
- Add tests for this transformation
- Remove support for Github archived-based building

test/test_trigger.py

@@ -1,7 +1,7 @@
 import unittest
 import re
 
-from endpoints.trigger import matches_ref
+from buildtrigger.triggerutil import matches_ref
 
 class TestRegex(unittest.TestCase):
   def assertDoesNotMatch(self, ref, filt):