Send notification to relevant users when a tag is added to or removed from a card

The need

When a tag is added to or removed from a card, it can certainly be of interest to the people concerned by the card (if the card is therefore assigned to them) to know that a tag has been added or removed. This is as valuable and interesting as knowing whether a comment has been made.

The suggestion

(and so on for push notifications and "Activity" app log.

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Additional context
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No files or tests are named. Start by locating how tag changes, push notifications, and Activity app entries are handled, then determine which users assigned to a card should be notified. Done means tag additions and removals notify the relevant users through the requested notification channels.