Add feature flag to stop calling Slack APIs

Reuben Pereirarequested to merge
rp/when-slack-down into master

What does this MR do and why?

Describe in detail what your merge request does and why.

When the slack_down feature flag is turned on, release-tools will stop making calls to Slack APIs. This can be useful when Slack is down or calls to Slack are expected to fail for any reason.

Not making calls to Slack allows release-tools to work when Slack is down.

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  • Has documentation been updated?

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