[api] add routes to get XML view of comments

issue 545
[api] test case for set_bugowner for groups
[api] implement delete_comment
[api] implement post to request comments
[api] remove package and project from event subscription

It was never used and we came to the conclusion we can't and don't
want to support per package subscriptions, but make use of filters
for the same - it's more powerful
[api] test that previous commenters get mails
[backend] improve rename content.new error message
[api] test case for set_bugowner for groups
[api] include people mentioned with @<login> as commenters

This is kind of faking, but I don't want to have yet another receiver role