Add JSON queries to the tabs on the GUI front-end.

These queries refer to tables 'jorders' and 'jlineitems' which will
be populated by as-yet-unpushed patches to server.py and bin/machdata
and dbgen. However, this front-end patch is almost entirely independent
of those changes (which are coming soon).
Attempt to make machdata demo do JSON queries.

Summary:
I've tested the following commands on my local machine (running with one leaf).

    machdata/bin/machdata setup
    machdata/bin/machdata rt
    machdata/bin/machdata query

    machdata/bin/machdata setup --json
    machdata/bin/machdata rt --json
    machdata/bin/machdata query --json

The new --json option sets up the "orders" and "lineitem" tables to contain a single
"data" column instead of a bunch of different columns; columns which have indexes
in the TPC-H version have persisted computed columns in the JSON version. I've left
the "orderkey" column as a regular BIGINT in both tables, since I figure the primary
key of a table probably shouldn't be a computed column in the real world.

Test Plan: manual

Reviewers: ankur, nikita, carl

CC: engineering

Maniphest Tasks: T7457

Differential Revision: http://grizzly.memsql.com/D3840
Add a JSON toggle to the frontend "Machdata SQL Client".

Toggling the "Toggle JSON Writes" toggle will tell the child aggregators
to start or stop "dbgen -j", sending JSON inserts to the master aggregator.
This is exactly analogous to "Toggle Write Workload".

Notice that you can have both write workloads going at once. This is
supposed to work fine.