Add "Subscribe for updates" call-to-action at the end of posts

[dsernst]

Closes #17

This adds a call-to-action at the end of blog posts to "Subscribe for updates" by email. This sends the email address to the API and sends the Welcome Email if it's not a duplicate.

• Has a success animation
• Has error handling animation
• Also tweaks the < Back home button at the very bottom of posts to more closely match the Subscribe button's style

Old:

New:

[jaredscheib]

Looks great -- thank you

The buttons' colors do appear slightly different because of the background that they're set against, but this is minor

Also, on "subscribe", we seem to lose 1px of margin below the button, when it changes from button to text, so the rest of the page appears to come up by that amount