Use master-detail layout on tablet

[Carifio24]

The main thing that this app is in need of is to have a master-detail layout on a tablet, similar to what we have on Android. I think this is fundamentally a two-step process, but we'll see how it goes:

• Split the spell table functionality into its own view controller (currently, the base view controller handles this). This way the main view controller can be more of a general app/layout orchestrator, rather than having to be concerned with the table contents as well. Done in Spell table controller #68.
• Make the main view controller layout adaptive based on device size