Late bytes fixes and broadcast support

[megascatterbomb]

Changes:

• Introduces POST /close_session which clients can use to close a session and simultaneously pass the late bytes through the request body. For legacy compatibility, the GET /close_session remains fully functional.
• Introduces a broadcast system. A system administrator with write access to the postgres database can modify the broadcasts table. The contents of this table can be obtained by clients via GET /broadcasts. There are no auth requirements for this endpoint, and the broadcasts cannot be modified except by direct database modification.

An Alembic revision has been made to enforce schema on the broadcasts table.