Allow server moderators to move custom emojis and stickers between boost tiers

If your server gets boosted to a certain level, the expanded custom emoji slots are filled, and then the server falls down to a lower level, the emojis which were uploaded to slots unlocked by the highest boost level will become inaccessible. They cannot even be edited until the server is boosted again. Clearly, this is not ideal.

I propose that moderators with the "manage emojis" permission should have the ability to control which emojis are accessible at a given boost tier, to ensure that the most common and popular custom emojis are accessible at the lower tiers, while more niche or unpopular ones become accessible at the higher tiers. This would also allow server owners to place "boost celebration" emojis in high tiers, which can be used to celebrate a server being boosted, but cannot be used until the server reaches a specific boost level.

A similar, if not identical, system should also be implemented for custom stickers.