v0.3.0

• Added: mailtea.domains.update(id, tracking_subdomain: nil) removes a
  tracking subdomain. The domain's links go back to being served from the
  Mailtea host. Links in mail you have already sent point at the old hostname
  and stop resolving — there is no way to reinstate them. Only the query string
  drops nils, so the removal travels in the body as an explicit null; leaving
  the key out and passing nil are different requests. An empty string is
  neither: it is refused with tracking_subdomain_invalid.
• Changed: the MX row in records now reports what the last verify found,
  instead of reading pending on every request but the verify itself. A domain
  nobody has verified reads not_started.

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Released from mailtea-app/mailtea@fed0718.