Add helper functions for client API

The Razor::View module provides Sinatra helper functions to render database
objects as hashes, which can then be returned to the client as JSON.

By making this functionality available as helper functions instead of wrapper
classes, the functions have access to other helper functions like compose_url,
and to each other. This permits, for instance, policy_hash to provide links to
Razor::Data::Tag objects by calling tag_url.

As suggested by @daniel-pittman, this link-based API format allows clients to
retrieve linked objects without building a URL. The linked object format is
intended to conform to RFC5988.
Add client API endpoints (policies & tags)

The endpoints are /get/tags and /get/policies, for listing tags and policies
respectively.