Align back button and title in MessageForm component

[oandregal]

The <MessageForm> component can have a back button and a title, like this:

If you take a closer look, though, they aren't properly aligned.

How to test:

• clone the repo
• edit the file src/config/development.local.json by changing css_url from https://widgets.wp.com/happychat/ to http://localhost:9000/. This tells Happychat to use the local CSS, not the one we distribute over our CDN.
• execute npm install && npm start
• introduce some text in the text fields and click the submit button and you'll see the <MessageForm> component.

[oandregal]

@evilluendas could I summon your magic CSS skills to help me with this?

Unlike Calypso, we don't have the <HeaderCake> component here. If this turns out not to be an easy fix I can look into pulling that component in.