add cancel button and escape key binding for add feed popover

The add feed popover had no convenient way to cancel. This patch adds a cancel button, plus a keyboard shortcut using the escape key. Because the key binding has to work inside the form text input, that text input has the `mousetrap` class. All other key bindings are unbound when the popover is visible, then rebound when it closes.

Note that I am not in a position to minify the JS or CSS, so I did not check in updates to those files.