Add support for /.well-known/change-password

[rfk]

Ben pointed out this neat feature of the iCloud Keychain where, if you go to change a password for a site through the keychain UI, it will load up /.well-known/change-password on that site. This gives the site a change to direct the user to the correct UX for changing their password, as described here:

https://twitter.com/rmondello/status/1009495517494173697?s=20

GitHub is an example of a site that supports it:

>>> requests.get("https://github.com/.well-known/change-password", allow_redirects=False)
<Response [301]>
>>> _.headers['Location']
'https://github.com/settings/admin'
>>>

And it seems like there would be little downside to implementing it on accounts.firefox.com. IIUC we just need a route at https://accounts.firefox.com/.well-known/change-password that redirects to https://accounts.firefox.com/settings/change_password.

No urgency on this, so I'm putting good-first-bug on it and putting it in the backlog.