Feature: Add Pokelevels to app somewhere

[magroader]

https://discordapp.com/channels/527605327303147520/562710992753131521/740232002745139322

As a user, I easily can see the sum of the levels my N highest level pokemon, where N is my pokeslots.

The app may need to refresh this value when pokemon are added, are removed, change levels, and when pokeslots change. It might be a good idea to maintain a list of pokemon IDs sorted by level to make this faster to calculate.