Refactor User schema constraints

[Nico-Sanchez]

If User works for every game, it shouldn't have:

• null constraint in experience and level.
• kalina_tree_level is required
  • All of above are champions-only fields.
• username should be provided from client, not autogenerated.

[lotuuu]

I'm touching some Kaline Tree stuff in #593 so I'll be deleting the kaline_tree_level requirement!