Fix a slow query on channel timeline (#6663)

migration/1597893996136-ChannelNoteIdDescIndex.ts

+import {MigrationInterface, QueryRunner} from "typeorm";
+
+export class ChannelNoteIdDescIndex1597893996136 implements MigrationInterface {
+    name = 'ChannelNoteIdDescIndex1597893996136'
+
+    public async up(queryRunner: QueryRunner): Promise<void> {
+				await queryRunner.query(`DROP INDEX "IDX_f22169eb10657bded6d875ac8f"`);
+				await queryRunner.query(`CREATE INDEX "IDX_note_on_channelId_and_id_desc" ON "note" ("channelId", "id" desc)`);
+		}
+
+    public async down(queryRunner: QueryRunner): Promise<void> {
+				await queryRunner.query(`DROP INDEX "IDX_note_on_channelId_and_id_desc"`);
+        await queryRunner.query(`CREATE INDEX "IDX_f22169eb10657bded6d875ac8f" ON "note" ("channelId") `);
+    }
+
+}