Add a "unified" entity API

[leafty]

Currently, there is no unified API for namespaced entities. The current API supports a unified API for namespaces, see GET /api/data/namespaces/{namespace_slug}; we should add a similar one for entities, e.g. GET /api/data/namespaces/{namespace}/entities/{slug}. The response would at least let the client know if the entity exists and which type it is (project or data_connector at the moment, maybe more in the future).