Slide in issue form on home page.

Closes #36.

Moves new issue form into the homepage, hidden by default.
Click on the button and it slides it in (toggling a `opened` and `closed` class on that button). -- @calexity will be using those to tweak button background images.

TODO: if the user doesn't have JS, just take them to a regular form page like before. Low priority TODO, IMO.