[Enhancement] Users should be able to get roles by clicking on reactions

[le-potate]

We should have a command to make a role self-assignable (or stop it from being self-assignable). This requires converting ?iam to use db. We should then be able to auto-assign roles from emojis on a message. There are different ways to go about doing this: I think a good idea would be to use the reply feature to reply on the message with the emoji and role, but support for use by message id could also be good.

This is pretty much the last feature needed to get rid of Mose. It also had a function to auto-assign a role to new users who join, but I've never heard anyone ask for this: Since this is related if its simple enough this could be implemented at the same time but otherwise it doesn't seem urgent and could be moved into its own issue instead once this is closed