Define an optional "launch request" URL for apps

[jmandel]

In addition to having an app expose its main URL in apps.json, allow apps to also define a structured "launch request" URL template.

Use case: external system (like a provider's EHR) that generates launch links for a given app, with expected/desired server + patient context embedded.

E.g. each app could expose a discoverable URL template with fields for provider (BB+ data holder server) and patient, like http://bpgrapher.org/launch/{provider}/{patient}

This would tell the app: try authorizing against the supplied server, for the specified patient. An expanded URL might be:

http://bpgrapher.org/launch/http%3A%2F%2Fbbplus-ri.aws.af.cm/123