Summary:
This allows a fork to be made by a user if:
* That user is an owner of the document being forked, or
* That user has full read access to the document being forked.
The bulk of the diff is reorganization of how forking is done. ActiveDoc.fork is now responsible for creating a fork, not just a docId/urlId for the fork. Since fork creation should not be limited to the doc worker hosting the trunk, a helper endpoint is added for placing the fork.
The change required sanitizing worker allocation a bit, and allowed session knowledge to be removed from HostedStorageManager.
Test Plan: Added test; existing tests pass.
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2700
Summary: This moves enough server material into core to run a home server. The data engine is not yet incorporated (though in manual testing it works when ported).
Test Plan: existing tests pass
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2552
