Split up User model by concerns

[Felk]

looking at the user model: https://github.com/TwitchPlaysPokemon/tpp-core/blob/master/TPP.Persistence/Models/User.cs
it already looks messy to me, even without all the commented out fields there is already a lot in there. Could there be a way to split it up a bit? I already see a few things that make this slightly complicated:

1. The entity currently mirrors the database representation. Either that needs to be changed too, or there need to be several repositories in the code exposing different objects that fundamentally just represent different fields of the same mongodb collection
2. some overlay events expect user information, and that may include everything. Right now the User model gets serialized straight to json and sent to the overlay, so having a dedicated User class for overlay events might be a good idea anyway. The question then becomes how assembling the data required for such an overlay event would work.
3. Most backend code will only need to see a user's "basic" fields, but the parts that do need to see specific fields would potentially need to make additional roundtrips to the database then.