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