Support "shareable"/public dashboards

[sheaphillips]

It should be possible for owners of dashboards within a project to share those dashboards with other users outside the project and/or the public. This is probably 2 or 3 different features, as the a "bearer link" could be part of the solution, and another would be truly public "discoverable" dashboard, and yet another would be a mechanism backed by StackButton's authorization mechanism.