ACL-based auth policy and the @permission_required_for_context decorator. For now used with /users API

[nhoening]

closes #201

• we might not do table-based authorization for a while (table-level auth: let data classes define which roles are allowed #200), as I did not find a nice way in this PR to do it, without having to separate acl lists
• I simplified how GET /users works - it can only get users per one account. Simplifying APIs is usually a good idea, I believe. It is a breaking change which I'll mention, but I'm very certain no-one relied on it yet.
• The users API does some column-based checks (authorize a request based on what fields are affected). We could even address this in our auth policy, I believe, but I don't see a clean way. Let's leave this to the endpoints for now. There should not be many more examples, maybe none (users are a bit special).
• We can still discuss what to do with service listings and /getService from here on out. They list required roles for each service. Will that persist?

[Flix6x]

Besides some minor fixes, I feel like we should be incorporating more Marshmallow functionality here.

And about your question:

We can still discuss what to do with service listings and /getService from here on out. They list required roles for each service. Will that persist?

I think this might be two questions actually:

• Will the USEF roles in those service listings become obsolete? I believe they might, but I haven't completely thought this through.
• Do we want the service listings to list relevant ACL? I believe we do.

[nhoening]

I believe the way we list auth access for services will be addressed a bit later. I might be interesting to put the ACLs in natural language, actually. I'll add a card to this project (https://github.com/SeitaBV/flexmeasures/projects/4).

[Flix6x]

Only small requests left: mainly some questions about documentation and the wish for a policy regarding raising errors or returning error messages in decorators.

[Flix6x]

Possibly still needs an adjustment to the function signature of the error handler.