[Bro] Bro issue tracking migrated to GitHub

[Jon Siwek]

Issue tracking for Bro is now handled on GitHub:

    https://github.com/bro/bro/issues

A selection of tickets have been migrated from the old JIRA tracker
based on criteria that goes something like "anything that is a
reproducible bug or simple enhancement".  Tickets left open in JIRA
will serve as a historical archive of Good Ideas and are only
modifiable to developers.  From now on, please create any new issues
on GitHub.  If there's an old JIRA ticket that has not been migrated
and you think it should, you can create a GitHub issue and simply
reference the JIRA ticket's URL.

A goal of the migration is to make it easier for people to contribute
by centralizing around a more familiar git development process and
also by improving the ongoing tasks of issue organization and
maintenance.

- Jon