Refactor Send page to use the WalletAssets service

! Depends on #1072 

As you can see from the resulting code, there's lot of things in WalletsAssets that (at least for me) make no sense to be there (i.e. all the conversion functions, placeholders, etc.)

But let's go step by step and not over engineer, the main idea of this refactor is abstract the coupled code to make it usable through all the app.