Redux migration to remove messageStore.threads[threadID].[lastPruned/lastNavigatedTo].

We don't **really* need this since messageStore.threads is persisted by both Redux and SQLite so the SQLite SET_MESSAGE_STORE will override the messageStore.threads that were REHYDRATED as soon as the app is first opened anyways, but figure it doesn't hurt.

Depends on D9517

messageStore.threads looks as expected after migration completes.