Manual mode for posting the Slack card

At the moment, a Slack card can be set up to be automatically posted when the PR/MR is either in draft or leaves draft status. A lot of teams will have their PR/MR be in a ready status but not want people reviewing it until they decide (for example once CIs have all gone green).

To support this use case, I’m adding the ability to set up a manual mode for posting which will listen for a comment e.g. “mm ready” (customisable by customers) to allow users to decide when the card is initially posted as opposed to it being done automatically.