Add --exclusive/-e to !reaction_roles

When reaction roles are set as exclusive, a user can only have 1
reaction role from that message. Others are removed automatically when
picking a role if needed.

backend/src/data/entities/ReactionRole.ts

@@ -19,4 +19,6 @@ export class ReactionRole {
   emoji: string;
 
   @Column() role_id: string;
+
+  @Column() is_exclusive: boolean;
 }