Easy way to get permalinks for messages in web archives

[ikedas]

Instead of the URL of the message on the web archive, which can change due to rebuilding archives, permalinks can be used. It is included in the Archived-At: field of the delivered message.

Expected Behavior

There should be easy way to know permalink.

Current Behavior

There is no easy way to know the permalink on the web archive.

Possible Solution

See related PR.

[ikedas]

Archived-At: field value includes message ID which could lead to personal identification. I'd be better to use its digest (The PR above includes this improvement).