dashboard: use Link instead of Reported-by

[a-nogikh]

Request from Theodore Ts'o:

<...> in fact, it might be nice if we could encourage upstream developers
put in the commit trailer:

Link: https://syzkaller.appspot.com/bug?id=5266d464285a03cee9dbfda7d2452a72c3c2ae7c

in addition to, or better yet, instead of:

Reported-by: syzbot+15cd994e273307bf5cfa@syzkaller.appspotmail.com

... and have Syzbot be able to translate from the Link: tag as being
equivalent to the Reported-by: link. That's becase the Link is going
to be much more useful to humans than the Reported-by --- we've had a
number of cases where as part of the patch review, we really wanted to
get back to the Dashboard page, and it's not easy to get to the
Dashboard from the Reported-by tag.

Should be very straightforward to implement. We can also make dashboard URLs nicer for this purpose:

https://syzkaller.appspot.com/b/5266d464285a03cee9dbfda7d2452a72c3c2ae7c

[HerrSpace]

We might want to grab a domain name for syzkaller, so we don't corner ourselves in with syzkaller.appspot.com, in case we ever want to get off app engine.

We could keep the main url there for now, but generate the Link trailers with something like syzbot.com and just forward to syzkaller.appspot.com.

[dvyukov]

If we have a custom domain, we could use it for web and potentially outgoing emails:
https://cloud.google.com/appengine/docs/legacy/standard/python/mapping-custom-domains

I think a custom domain is a good idea on its own. However, I think it's additional maintenance burden. So we need to ask @tarasmadan.

[HerrSpace]

I chatted with Taras and he pointed out this is probably to much work for now.

We can also just implement this FR with the current domain and should be ever move, we can deploy a redirector service on the current (then old) domain.

[dvyukov]

A related discussion on LKML:
https://lore.kernel.org/all/CACT4Y+brH-ygxHYzstC9dgFcQ=NnYgE48E4j+csE3JqH4FaM5w@mail.gmail.com/

Kernel maintainers want to see a link to lore in bug fixes.
Unfortunately we cannot include link to lore in reports (consequently cannot use it to link reports and fixes).
However, we can ask kernel developers to include link to lore in bug fixes.