Migration path for pre-existing WooCommerce accounts

User story: I am a returning customer. When I subscribe, I want to use my existing shop account, so that I can see all my orders in one place.

Suggested solution: Custom registration page for existing WooCommerce users prompting for:

• Existing address
• Existing passphrase
• Librem One address
• CAPTCHA

Given there is an existing account
And the Librem One address is available
When the user enters the correct credentials
Then create a gratis Librem One account
And set the recovery email to the old address
And update the WooCommerce account details

...the user can now migrate to a gratis account, and upgrade to a paid account if they want to.

Notes:

We currently disallow recovery emails that match an existing WooCommerce account. Replace this error message with something like:

This recovery email is in use at the shop. Please migrate your account.

Additionally, we should offer a migration link from within WooCommerce. (New ticket once this one is complete.)