Bug#1009181: File suffix for message mbox links should be .eml

Package: debbugs-web

- Open any bug page and copy "mbox" link for particular message (not 
mbox folder for the whole discussion).
- Check "content-disposition" header in server response to such request

curl -I 
'https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1009036;mbox=yes;msg=5'

HTTP/2 200
date: Fri, 08 Apr 2022 10:04:43 GMT
server: Apache
cache-control: public, max-age=86400
content-disposition: attachment; filename="bug_1009036_message_5.mbox"
x-content-type-options: nosniff
x-frame-options: sameorigin
referrer-policy: no-referrer
x-xss-protection: 1
permissions-policy: interest-cohort=()
strict-transport-security: max-age=15552000
etag: 95464ed9b7121e98437a0a1c28d57650
x-clacks-overhead: GNU Terry Pratchett
content-type: message/rfc822

Notice that the default file name to save the message has ".mbox" suffix 
while freedesktop mime database entry assumes ".eml" for the 
"message/rfc822" mime type:
https://sources.debian.org/src/shared-mime-info/1.10-1/freedesktop.org.xml.in/#L5466
As a result if "message/rfc822" mime type is associated with 
thunderbird.desktop file then the application starts composition of a 
new message with the .mbox file as attachment instead of displaying the 
downloaded message.

I have not managed to convince thunderbird developers that there should 
be a way to tell the application that some file should be opened namely 
as a message no matter what is the name and the extension of the file. 
They believe that current heuristics is correct and ".mbox" suffix means 
collection of messages in "application/mbox" mail box. Thunderbird does 
not support opening a mbox file outside of its mail directories.

While I am still thinking that such behavior is partially a thunderbird 
fault, I suppose, debbugs should follow conventions and use .mbox file 
suffix only for downloading of the whole discussion.

Single message should be saved to an .eml file.