[ML] Support legacy watcher URLs to the Anomaly Explorer page

[darnautov]

After improvements around the persisted URL state in ML plugin #83033 the structure of the Anomaly Explorer URL has been changed, in particular, the page URL state is not under _a key directly, but _a -> explorer instead.
We need to support watcher URLs created prior 7.11, so the following is required:

• Adjust the email template according to the new URL structure
• Create a redirect rule or support legacy URL shape to make sure overall swim lane cell selection is restored from the URL

[elasticmachine]

Pinging @elastic/ml-ui (:ml)