Extract modal stuff into another SCSS file.
New modal infrastructure using a ModalController.

The intent with this is that there's significantly less duplication
between the help modal, the future settings modal, and everything else.
Get rid of a `debugger`.
Use vex.js for the hotkeys modal. It's cleaner.
Move settings panel into a vex modal as well.
Focus message when replying with empty instance.

The new behavior is to focus the message field when replying to a zephyr
with an empty instance, but to focus the instance field when there's no
defined instance (filtering to a class). This fixes #44.