Improve efficiency and UX of unique reactions checks

[eibex]

Currently, for each reaction a message has we are checking all users who reacted to it.
This is extremely slow on messages with many reactions and/or people who reacted to them.

A possible solution could be to cache reactions, however, it could lead to problems if the DB isn't in sync anymore due to manual changes, outages, errors, etc.

reaction-light/cogs/roles.py

Lines 79 to 87 in f6445e1

	for existing_reaction in msg.reactions:
	if str(existing_reaction.emoji) == reaction:
	continue
	reaction_users = await existing_reaction.users().flatten()
	reaction_users_ids = (reaction_user.id for reaction_user in reaction_users)
	if user_id in reaction_users_ids:
	await msg.remove_reaction(existing_reaction, user)
	# We can safely break since a user can only have one reaction at once
	break

[eibex]

Potential solution could be to check user roles instead of each reaction (and each user per reaction) and try to remove the associated reaction (assuming it wasn't manually added).