Add a progress bar when chart/table data is loading

[cstephen]

Closes #48.

This PR adds a progress bar below the place selector for all applicable ARDAC items (all ARDAC items that fetch data from the API for a chart or table).

To test, select a location to generate a chart for several different ARDAC items. Observe that:

• The progress bar is only visible while the app is actively trying to fetch data from the API.
• The progress bar disappears as soon as the chart or table loads.
• If there's an error (like an invalid location), the progress bar disappears as soon as the error is detected.

[brucecrevensten]

This works! I think it's a good idea to have it watch the actual HTTP status but we can do that in the future (#114 )