Add inline newsletter call-to-action signup

[dsernst]

Coinbase sets a good example:

^ from their front page

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^ footer of every post

[dsernst]

Love the message on this one from dcurt.is

[jaredscheib]

We've done the frontend for this: https://github.com/liquidvote/blog/commit/2df551dd25d900bbaed0d48dc527bb9a3e7ddf90

We still need:

• [x] api
• [x] send to api
• [x] show success
• [x] maybe some error handling
• [ ] maybe some client-side validation

[dsernst]

Sends to API as of https://github.com/liquidvote/blog/commit/cdfa961188445a6dfa887e96de0dcf67c9994884

[dsernst]

Added success message in https://github.com/liquidvote/blog/commit/d05d99b9d3e4475b25c91ff43f8bcf88e3f41582:

[dsernst]

Right now there's no feedback if there's an error

[dsernst]

Added error handling:

• If the email is a duplicate (status code 409), show the Subscribed confirmation anyway.
• If the email is invalid (status code 400), add a 1px red border around the input field.