send_reaction: allow passing None to remove reactions from a message.

[nitanmarcel]

send_reaction should allow to pass None to remove reactions to a message.

Added an use_default argument that can be used with reaction=None to use one of the default reactions

[nitanmarcel]

Actually I wonder if use_default should override reaction if set to True

[Lonami]

I'm going to close this since v2 is currently stalled until I decide to revive it.