Configurable 'send to channel' flag for slack reminders from alerts

### Problem Statement Right now when we are using sentry alerts to send slack alerts to a particular channel it sends the initial alerts and follow up alerts as thread replies to the initial alers, which is nice and makes sense to reduce noise but during the thread reply, send to channel option is also done meaning we get a message in channel as well as the thread, this does the exact opposite of noise reduction due to the sheer number of alerts we get on a day to day basis. If we were to have this feature then sentry alerts would have been chosen as our sentry escalation and reminder system. ### Solution Brainstorm During the configuration of sentry alerts to be send to slack, keep this a flag we can control in order to determine if a alert is to be sent to just the thread or thread+channel as well. ### Product Area Alerts