Centralize periodic checks for mutes, tempbans, vcalerts, reminders, and scheduled posts

This should result in a significant performance improvement.
The new method is also more precise than the old one, allowing
the aforementioned checks to be performed with second-precision.

backend/src/migrations/1632582299400-AddIndexToRemindersRemindAt.ts

@@ -0,0 +1,16 @@
+import { MigrationInterface, QueryRunner, TableIndex } from "typeorm";
+
+export class AddIndexToRemindersRemindAt1632582299400 implements MigrationInterface {
+  public async up(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.createIndex(
+      "reminders",
+      new TableIndex({
+        columnNames: ["remind_at"],
+      }),
+    );
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.dropIndex("reminders", "IDX_6f4e1a9db3410c43c7545ff060");
+  }
+}