Capture and indicate channel promotion/demotion activity

[cnunciato]

Particularly with core plans, it can be helpful (particularly in debugging/incident-related scenarios) to know when a package was promoted to the stable channel (or perhaps any channel, for that matter). We should track this activity and represent it in the UI.

Not auto-backlogging this in order to prompt some discussion first in triage. 😄

[chefsalim]

We already track the promote/demote activity in the audit logs for debugging/incident purposes. Exposing it in the UI might be interesting if/when we have notification capability (eg, "a package you depend on has a new version promoted to stable").

[dmccown]

This would be more useful as part of notifications #839

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