Support automatically creating recurring events

Also implement locks for tasks that can have unintended side-effects if ran simultaneously.

Other minor UI tweaks.

lib/interfaces/locker.ts

@@ -0,0 +1,36 @@
+// This is a simple, in-memory lock, which works well enough if you're serving from a single source/server
+
+function wait(milliseconds = 100) {
+  return new Promise((resolve) => setTimeout(() => resolve(true), milliseconds));
+}
+
+const expirationInMilliseconds = 15 * 1000;
+
+const locks: Map<string, { expiresAt: Date }> = new Map();
+
+export default class Locker {
+  protected lockName: string = '';
+
+  constructor(lockName: string) {
+    this.lockName = lockName;
+  }
+
+  public async acquire() {
+    const currentLock = locks.get(this.lockName);
+
+    while (currentLock) {
+      // Only wait if the lock hasn't expired
+      if (currentLock.expiresAt > new Date()) {
+        await wait();
+      }
+    }
+
+    locks.set(this.lockName, {
+      expiresAt: new Date(new Date().setMilliseconds(new Date().getMilliseconds() + expirationInMilliseconds)),
+    });
+  }
+
+  public release() {
+    locks.delete(this.lockName);
+  }
+}