Allow reducing the number of Slack buttons on every alert-group post

[tyrken]

What this PR does

Currently the initial Slack post for a new Alert group contains two rows of buttons, which takes up a lot of screen real-estate besides being a bit daunting to new users. So I've added a FEATURE_FEWER_SLACK_BUTTONS to remove several buttons, chosen a bit arbitrarily by me on our expected usage:

• Responders - we expect engineers to accept alerts via Ack rather than a manager distributes work via assigning tasks, so don't think we need this. And you can add Participants via the OnCall plugin UI.
• Attach to - we don't expect to use this
• Format Alert - this is only needed for initial integration setup, not long-term
• Add Resolution notes - hide if no notes added yet (we don't expect to use this, can be done via Slack context menu if needed)
• Unresolve - I don't think this is needed often (and can be done via OnCall plugin UI) and removing this deletes all buttons for Resolved events, which is nice both from a reduced size and the reduction in importance compared to still active alert groups.

Which issue(s) this PR closes

This doesn't fully resolve #4165 but would do most of it together with another PR I'll be opening shortly.

Checklist

I've not added any extra tests/docs yet, awaiting feedback on whether this feature is acceptable & in what form you'd like extra work before doing so.

• Unit, integration, and e2e (if applicable) tests updated
• Documentation added (or pr:no public docs PR label added if not required)
• Added the relevant release notes label (see labels prefixed w/ release:). These labels dictate how your PR will
  show up in the autogenerated release notes.

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