Add an imagery blacklist to capabilities

This blacklist is for imagery which is of an incompatible license or ToS but is incorrectly used with some frequency.

This adds a <policy> element to the capabilities XML which contains the blacklist information within it.

@tomhughes, could you deploy this branch on errol for testing?

Also, when doing so could you add ``- ".*\\.stamen\\.com/.*"`` to application.yml? We need something blocked to test with, and stamen has layers that are unlikely to be in any presets or used.

Fixes [trac 5024](https://trac.openstreetmap.org/ticket/5024)