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/data/Mutes.ts

@@ -0,0 +1,23 @@
+import moment from "moment-timezone";
+import { Brackets, getRepository, Repository } from "typeorm";
+import { Mute } from "./entities/Mute";
+import { DBDateFormat } from "../utils";
+import { BaseRepository } from "./BaseRepository";
+
+export class Mutes extends BaseRepository {
+  private mutes: Repository<Mute>;
+
+  constructor() {
+    super();
+    this.mutes = getRepository(Mute);
+  }
+
+  async getSoonExpiringMutes(threshold: number): Promise<Mute[]> {
+    const thresholdDateStr = moment.utc().add(threshold, "ms").format(DBDateFormat);
+    return this.mutes
+      .createQueryBuilder("mutes")
+      .andWhere("expires_at IS NOT NULL")
+      .andWhere("expires_at <= :date", { date: thresholdDateStr })
+      .getMany();
+  }
+}