Allow shell-style subscription patterns

- Allows users to subscribe to shell-style patterns like 'he.*' or
  '*.wikivoyage.org'.
- Reduce the maximum subscription quota to 10. 100 should be fine too, but I'd
  like to err on the side of being conservative and wait to get some data about
  how people are using the service.
- Maintain the subscriptions in sort order, with shorter subscriptions ahead of
  longer ones and wildcard subscriptions ahead of exact (because short,
  wildcard subscriptions are the most likely to match).

Change-Id: I4e9d3dede3711c99c6dae9f351dca6bea0d88acb