Basic theming support

This adds a new option, CUSTOM_IMAGE, to config.py. It defaults to None.

If you copy an image into static/i (say my_logo.jpg), and then set CUSTOM_IMAGE to "my_logo.jpg", it displays that image instead of the SecureDrop logo.

If you're using a custom image, it also adds a smaller "Powered by SecureDrop" under the logo.