feat: Implement post search.

Related tickets:
* [TNL-8428](https://openedx.atlassian.net/browse/TNL-8428)
* [BB-4418 (OpenCraft Internal)](https://tasks.opencraft.com/browse/BB-4418)

src/discussions/posts/data/thunks.js

@@ -60,6 +60,9 @@ export function fetchThreads(courseId, {
   if (filters.status === PostsStatusFilter.UNREAD) {
     options.view = 'unread';
   }
+  if (filters.search) {
+    options.textSearch = filters.search;
+  }
   return async (dispatch) => {
     try {
       dispatch(fetchThreadsRequest({ courseId }));