Fix the way new comments get posted

[joshsmith]

Right now when a comment gets posted, it just looks like the preview is being generated, and the button state does not change at all. We should fix this and make the experience a lot smoother.

[joshsmith]

Also appears to be the case that the preview is changing height or something.

[joshsmith]

Here's a video that demos the bugs that need fixed here: https://s3.amazonaws.com/f.cl.ly/items/413H2P1S16351W3U0y0r/BugDemo.mov

The height changes slightly (flickers) when previewing and submitting, and the comment form does not change back from the previewing state.

[joshsmith]

The bug, on a particular post:

• Write something.
• Hit preview button.
• Hit comment submit button.
• The comment will submit but the preview button remains highlighted and the preview remains there.

Expected behavior:

• Write something.
• Hit preview button.
• Hit comment submit button.
• The comment submits, the button changes to "Write", and the preview is cleared.