Add data downloads to the web app

[dajtxx]

It would be very useful to be able to download messages from the physical_timeseries table via the webapp.

Stage 1, add a button and dialog to download the messages over a date range from the logical device page.

Stage 2, allow multiple devices to be selected from the logical device list and then show the dialog for date range and download the data for all of those devices.

[dajtxx]

Device names needed escaping. See commit 1f22a16.