"Team" entity refactor

[sei-bstein]

We talk about this a lot internally, but I noticed we don't have an issue explicitly dedicated to it.

Currently, teams get an ID, but are not actual entities - they don't have a table and are thus not subject to typical relational constraints that can be useful for system stability, efficient queries, etc. This has caused a number of bugs over the years, and results in some pretty gnarly workarounds that result in DB denormalization.

Proposed solution is to:

• Make Team a first-class entity
• Move relevant fields from Player to it (session times, scores, challenge completion, team name, whatever else makes sense)
• Create a FK constraint from player to team
• Refactor to take advantage

This is a large lift but is really important for longterm application health. We'll address when it makes sense.