Add data downloads to the web app

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.

Start by locating the logical device page and logical device list, then trace how messages are read from the physical_timeseries table. Define the date-range download dialog and single-device flow first, followed by multi-device selection. Done means both stages let users download the requested data from the web app.

Written by the indexing model from the issue text.