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/migrations/1575145703039-AddIsExclusiveToReactionRoles.ts

@@ -0,0 +1,19 @@
+import { MigrationInterface, QueryRunner, TableColumn } from "typeorm";
+
+export class AddIsExclusiveToReactionRoles1575145703039 implements MigrationInterface {
+  public async up(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.addColumn(
+      "reaction_roles",
+      new TableColumn({
+        name: "is_exclusive",
+        type: "tinyint",
+        unsigned: true,
+        default: 0,
+      }),
+    );
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.dropColumn("reaction_roles", "is_exclusive");
+  }
+}