Add backend support for cross-team collaboration

[ryukoposting]

• Add a new column shared to the events table.
• When two teams using the same event_code and secret_code both have their shared flags set to true, the /team/{teamNumber}/gameYear/{gameYear}/event/{eventCode} endpoint will return the combined match data of both teams.

Note some important behaviors:

• Match data is shared, not notes or inspections (will want to share inspections too, but never notes)
• Teams with shared=false cannot see data from teams with shared=true. In other words, you can't benefit from scouting collaboration unless you're also collaborating.