Conversation ID now gets set from initial notice.

Conversation ID now gets set from initial notice.

This will work without much extra effort because there will always be
more notices (higher value) than conversations (so no collisions).

But please run upgrade.php to avoid having an autoincrement id on
conversation table.

Installations using code after 2014-03-01 will have identical
conversation IDs to the initial (conversation root) notice IDs. This
will not affect older installations, which will have very different
values.