WIP: add send email button

[thiagoarrais]

The purpose of this pull request is to add a "send email" button to the editor.

When this is ready, it should work like this:

1. User composes heml message
2. User chooses "send"
3. An email field popups up
4. User enters email and hits "send" again
5. Message is rendered to HTML and, through the magic of SparkPost, arrives at the user's inbox

I've started by making the email field show up. It is still a bit ugly since I haven't figured out how to properly style it.

There is a live version of my changes at http://magical-plot.surge.sh/editor

Next steps will be:

• make the active tab marker work properly
• style the email field
• send the content to the API server
• server-side rendering + sending email

I'm learning React. When reviewing please feel free to point out any non-idiomatic usage of React. It'll help me a lot