Write workflow to announce new bug issues on slack channel

[nabramow]

We decided in the meeting we don't want to autoadd tickets to backlog. Rather we want to make a new Slack channel and announce new bugs there.

This ticket involves:

• Make new slack channel for bugs (similar to how we have a github-integrations channel). Maybe call it #bugs-needing-triage?
• Write github workflow that triggers when a github issue gets the tag 'bug: needs triage' added and announces them in the new slack channel. Should include the link to the github issue and, if possible, the description in text so people can see it at a glance.

Just FYI the process will be that if people look at a bug there, they can decide to add it to the backlog, or if urgent add it to couchers engineer ready column, or mark as closed if it's not legit. Similarly, could be moved to Couchers Ops if the ticket signals better documentation is needed, better design, etc.