* FEATURE: Add logging for CustomEmoji
We didn't provide any logs for CustomEmoji before, nor did we record the
person who added any emoji in the database. As a result, the staff had
no way to trace back who added a certain emoji.
This commit adds a new column `user_id` to `custom_emojis` to record the
creator of an emoji. At the same time, a log is added for staff logs to
record who added or deleted a custom emoji.
锦心