Remove Pocket indicators in UI and disable it by default

We should look at disabling Pocket as it is now integrated into Firefox directly. Might be enough just flipping extensions.pocket.enabled`.

(FPI might actually break the Pocket login flow and Mozilla recommends disabling it, so that it probably reason enough to disabled Pocket for the time being: https://help.getpocket.com/article/1046-trouble-saving-to-pocket-stuck-in-a-loop)