Validate imports before exporting

[shnizzedy]

If a user imports JSON, the relevant key-values are rendered, booleans are handled as truthy, and irrelevant key-values seem to be ignored (that all seems good to me). Then if/when the checklist is exported, the original non-validated inputs are included (extra keys, non-boolean truthy values). We might want to drop extra keys
and coerce types, either on import or before export. Or we might want to actually validate and reject JSONs that don't match the schema.

Originally posted by @shnizzedy in #44 (review)