Rewrite emoji stats

[stepech]

At the moment, emojis are saved the moment they are used as reaction, and they are never deleted. Additionaly, all stats are displayed from the time the emoji was first used, making some stats about short-lived but viral emojis misleading. Like the situation with :okayge: which was used 215000x leading to usage per day as high as 500, even though this trend is long dead.

Proposed changes:

• don't add emotes first time they are used. Keep the database synced with guild. Listen to emoji add and removal events and update/delete corresponding entries. Additionaly full emoji list could be synced the moment bot starts up - potentially expensive, but just once in a while.
• Introduce some way to limit lifetime of no longer used emojis. Save stats just for some period
  • like dropping entries older than 1yr
  • substracting 10% of total emoji used count at the end of every month - that would target mainly emojis that have numbers way above their real usage, meanwhile used emojis would easily overcome this number
  • etc.
• More variety to emoji commands? If we modify it in a way that supports recognizing when were which emojis incremented, we can have separate stats for year/month/day. Additionaly if we include this as a slash command and make responses ephemeral for normal users/public for mods, we can make this command available to everyone (In case spam is the reason it is not done yet). Seems like a frequently requested feature.