Add a field for extra report notes on game session from a DM in the GameSession endpoint

[ivellios]

We would like to be able to store extra information on the stuff that happened during the game session. Especially rewards for players, some special effects that took place or character has some long-term condition.

Each DM should be able to add a note for the game when filling out the report from the game.

The first step for this feature is to add proper field in the backend models.

Model: GameSession
Field name: report_notes

Also add proper db schema migration.