Summary:
1. The /import endpoint wasn't handling poor names like ".grist" as
intended, instead trying to import them using the plugin-based imports.
2. The SaveCopy dialog was allowing users to save to an empty name,
particularly bad because new docs now default to an empty name.
Error manifested as "Cannot parse data" to the user.
Reported in https://secure.helpscout.net/conversation/1242629116/292
Test Plan: Added tests for both parts of the fix.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D2573
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
