Cleanup "REST" API

[mattwelborn]

The Fractal API, while very useful, is not exactly best practices. We should clean it up in a number of ways, including:

• Pluralize collection endpoints, e.g. /molecules, /keywords
• Add access to resources by ID, e.g. /molecules/1234. Add corresponding methods to Portal, such as FractalClient.get_collections(id).
• Return appropriate status codes, such as 204 after DELETE or 201 after PUT. (And change Portal to correctly check status code.) 401 (unauthorized, i.e. not logged in) and 403 (forbidden, i.e. logged in with insufficient permissions) should be used correctly. Also, we return 401 for invalid data blobs (should be 406?).
• Painful, but GET methods should probably not have bodies. (Unclear if this is allowed or not. See e.g. Proposal: Support GET bodies whatwg/fetch#83)
• Going forward, we should also consider implementing new functionality following REST best practices. For example, adding entries to collections should be a PUT request.

REST guides:

• https://medium.com/better-programming/restful-api-designing-guidelines-the-best-practices-39454135f61
• https://www.restapitutorial.com/

@doaa-altarawy

[doaa-altarawy]

I'm fully supportive of this refactor.
GET without a body.
I'm okay with having just POST for add or update, and no need for PUT, as many ppl does with REST.