Content Warning for the game session

[ivellios]

Apart from Extra Notes on the Game Session, we would like to store and display extra information with Content Warning specifically. This information should be stored, edited and displayed exactly as Extra Notes, apart from being simply separate, so it has better exposition to the user.

Feature: Content Warning information for the users

Scenario: DM can add Content Warning
  Given I am a DM
  When I browse to the page for booking a game
  Then I can fill separate field for Content Warning placed under Extra Notes

Scenario: A player can see Content Warning on the Game Session Page
  Given I am a Player
  When I browse to the page with a Game Session
  And the game session has Content Warning filled out
  Then I can see it as a section placed above the Extra Notes section

Scenario: A player cannot see Content Warning on the Game Session Page when not provided
  Given I am a Player
  When I browse to the page with a Game Session
  And the game session does not have Content Warning at all
  Then I cannot see any section with Content Warning

This story should be covered in both repositories: frontend end backend with separate PRs.