Looking up values from the `emojiStore` calls out to the browser's localStorage API and then decodes a JSON blob. This makes it relatively slow.
Previously we were doing this lookup in the emoji-picker's `init()` function, even if `isActive` was false. If many inactive emoji pickers are rendered simultaneously (e.g. for discourse-chat reactions), this performance hit quickly adds up.
This commit updates the service to notify about changes, and uses a computed property to provide a cached value in the emoji-picker.
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