Thread muting [APP-29] (#500)

* Implement thread muting

* Apply filtering on background fetched notifs

* Implement thread-muting tests

src/state/models/muted-threads.ts

@@ -0,0 +1,29 @@
+/**
+ * This is a temporary client-side system for storing muted threads
+ * When the system lands on prod we should switch to that
+ */
+
+import {makeAutoObservable} from 'mobx'
+import {isObj, hasProp, isStrArray} from 'lib/type-guards'
+
+export class MutedThreads {
+  uris: Set<string> = new Set()
+
+  constructor() {
+    makeAutoObservable(
+      this,
+      {serialize: false, hydrate: false},
+      {autoBind: true},
+    )
+  }
+
+  serialize() {
+    return {uris: Array.from(this.uris)}
+  }
+
+  hydrate(v: unknown) {
+    if (isObj(v) && hasProp(v, 'uris') && isStrArray(v.uris)) {
+      this.uris = new Set(v.uris)
+    }
+  }
+}