[api/silenced] clarify SILENCED_SUBSCRIPTION_URI and SILENCED_CHECK_URI regexps
[api/silenced] clarify SILENCED_SUBSCRIPTION_URI and SILENCED_CHECK_URI regexps

I think we should escape the hyphen here for purposes of indicating that we want
to match a hyphen and not to indicate a range.

portertech/sensu,sensu/sensu,somic/sensu