MM-38132: Improve Auto Responder logic (#18264)

We properly truncate the date part from the createAt time.

The SQL query is improved now to directly return the bool.

Fixed the test to properly test the feature.

https://mattermost.atlassian.net/browse/MM-38132

```release-note
NONE
```

store/storetest/post_store.go

@@ -3145,6 +3145,8 @@ func testHasAutoResponsePostByUserSince(t *testing.T, ss store.Store) {
 			Message:   "message",
 		})
 		require.NoError(t, err)
+		// We need to sleep because SendAutoResponseIfNecessary
+		// runs in a goroutine.
 		time.Sleep(time.Millisecond)
 
 		post2, err := ss.Post().Save(&model.Post{