Add buttons to report errors and leave feedback

Fix #7 (closed)

• I add a mailto button to the menu, which users can use to leave feedback.
• I wrap ImageViewContainer in an error boundary, so the app remains usable if a loading error occurs (i.e. users can select another dataset or retry to fetch the current one).
• I wrap the entire App in another error boundary. If users hit the retry button in this one, the page reloads.
• Both error boundaries include a button to report the error. It's a mailto link with a pre-filled subject and body. The body contains the error message and asks users to provide additional context.

Note that in production, users won't see Create React App's error modal.

While I'm at it, I also make sure that the currently selected dataset is highlighted in the file browser. This change is not visible in the GIF above, so here is a screenshot instead: