Support /.well-known/change-password

Mastodon should support the `/.well-known/change-password` de-facto standard https://mobile.twitter.com/rmondello/status/1009495517494173697 et al.

This would effectively be a 301 from `/.well-known/change-password` to `/auth/edit` for the mastodon Web UI.

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- [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
