Disable actions for auto-generated nodes/docs

[thseiler]

WHAT:
While working on the image PR, I noticed that it was possible to edit certain node types that StrictDoc cannot currently save:

• autogenerated documents and nodes like test reports
• nodes that were merged from source_code
  These changes appear to take effect initially (as the in-memory model is updated), but they are lost when the server is stopped.

WHY:
Ensures that the user notices that he can currently not edit these node types.

HOW:
The PR introduces an ActionPolicy object and hooks it:

• to the document action button (edit metadata)
• to the node action buttons (edit, clone, delete, add)
• to the toc (move nodes)
• to the individual fields in the edit form

Points for Discussion:

• The ActionPolicy is currently a proposal. I am not really sure whether the policy correctly handles all the edge cases, especially for merged nodes. But as the logic is centralised, it should be easy to adapt as needed.
• I noticed some issues when editing a merge node (where some fields are parsed from the source code and others come from the .sdoc). The origin metadata for fields would be lost on edit, and the UID / MID would not be written back to the sdoc, so that the link would be lost on subsequent server starts. I've tried to address these as well.
• I had some trouble with the SDocTestServer hitting the ReadTimeout, and noticed that the exit handler is not called when an exception happens in the enter block. I added an exception handler to clean up the server so that the end-to-end tests could continue.