Add notification and call to action when no transit data is found

[rmglennon]

We often test in areas where transit data is plentiful, but there are still areas without coverage.

To reproduce:

• Pan to the ocean...or anywhere else known without transit data.
• Click Routes or any of the transit buttons

The UI looks the same as it would if transit data existed, but the drop-down boxes are empty. You need to click the lists to learn that no results were found

We could use this as an opportunity to show a message with a call to action that is something like, "We don't have any transit data for this area. Try searching for another location. If you know of an open transit dataset, please submit it to the Transitland Feed Registry." with links to the submission docs.