Add "only encrypted emails" filter

[sosnovsky]

There was a customer request to have ability to show only encrypted emails.

Let's add show only encrypted messages filter to the search screen, tapping on it will show only encrypted messages on search results screen.

Earlier similar functionality was implemented on Android, but was disabled as there were some issues - FlowCrypt/flowcrypt-android#715

I think we can use users.message.list endpoint with q parameter for filtering encrypted messages.