Refactor VVM into MVVM

Change-Id: I15a12f0474faf4acbcfe87676aee5f64c17910a9
Fix regression bugs from MVVM refactor

Change-Id: Ibe6dc40bc2964c94a0acdd4780e78ba2538eb7c7
Add support for pronounciation audio

Change-Id: Ieab1fd7a87ce4a09b493c4a3fab310e30a062d93
Implement Leitner system for spaced repetition

Change-Id: Iea7ce54eef1e195626913454c73845ae665c7292