[commhistory-tool] Add import-json command

The JSON format is editable and simple, allowing easy import of
customized data. Export support can be added as well for convenience.

Unlike the binary import/export format, no attempt is made to preserve
IDs or other specific details, in favor of simplicity and editability.
[commhistory-tool] Add -groups and -calls options for deleteall

deleteall also now deletes via models first, so that running processes
will be updated.