[Feature Request] Allow per user/organization i.o. images

[anjavangestel]

Problem description
As a node I can allow certain images to be executed on my data.
Anyone in a collaboration then can execute these.
Is there also a way I can allow a certain user or organization to use (whichever image on) my data?

So instead of saying 'I trust this image', I want to be able to say 'I trust this person', or 'I trust this organization'.

[bartvanb]

To be done:

• POST the permissions set in the node config file to the server so that everyone can see that a task will not be accepted. This requires work done in request allowed images #12
• Implement a check before starting task that checks permissions
• Before starting a task, check if user has permission on all relevant nodes (as far as you can see when they are online).
• If nodes are offline when task is started, we cannot check permissions. Then, yield a clear error if user was not permitted.
• Implement the same strategy for organizations
• Update docs (waiting to do this until Change/reorganize user docs #537 is merged)

Implementation is on branch feature/allowed-users-and-orgs-for-task