Add "share" button to mailbox popup

[ranatalus]

Related to #16

At the bottom of the page, next to the text box displaying the permalink URL for a given box, add a "Share" button that has the following behavior:

Desktop OS: Clicking the box copies the URL to the clipboard, and displays a small visual to indicate this (perhaps a bubble that says "Copied!" rising out of the button)

Mobile OS: Calls the OS share dialog to share the box URL