Add a "block-all new-source" hint

This hint will block all "new" source migrations.  Source migrations
for packages already in testing will be affected by this.  As usual,
this hint can be overruled by an unblock hint.

Closes: GH#8

Signed-off-by: Niels Thykier <niels@thykier.net>

britney.py

@@ -1443,7 +1443,8 @@ class Britney(object):
                 blocked['block-udeb'] = hint
                 excuse.add_hint(hint)
         if 'block' not in blocked:
-            for hint in self.hints.search(type='block-all', package='source'):
+            for hint in self.hints.search(type='block-all'):
+                if hint.package == 'source' or (not source_t and hint.package == 'new-source'):
                     blocked['block'] = hint
                     excuse.add_hint(hint)
                     break