Support limiting users to one reaction

[eibex]

Describe the PR changes

• Adds a new step in the message creation process asking if the message needs to have mutually exclusive roles or not
• Adds new limit_to_one column to messages table
• Adds check in on_raw_reaction_add() if a user has already selected a reaction (and the message is marked as unique), if so the reaction is removed, which causes on_raw_reaction_remove() to trigger, leading to the removal of the role

What could be better
Changing the "unique" status with the rl!edit command. The problem here would be iterating through existing reactions and arbitrarily removing all of them but one per user. I just think it's easier for admins to delete the message and recreate it.

Known Issues
If you select multiple reactions really fast all of them will get removed. This is due to the API latency, cannot circumvent this.

Closes #44

[eibex]

I will merge this next week since I released 2.3.0 a few hours ago, I didn't think this PR would be so quick