Preventing late joining betas from interrupting an in-progress game

#### What's this PR do?
Adds a flag to mark a game as having a started. If a beta tries to join a game in progress, then that person will receive a message informing them of that. Players currently in the game will not be disrupted in any way.

#### Where should the reviewer start?
competitive-stories.json
`beta_joined_game_in_progress`: This is the opt-in path ID to send joining betas to if they tried joining a game already in progress.

sgCompetitiveStory.js
Added a `game_started` boolean to the schema that defaults to false when the doc is created.

SGCompetitiveStoryController.js
`game_started` is set to true when a game is about to start. When a beta joins, if that value is found to be true for the game, they're given the `beta_joined_game_in_progress` message.

#### What are the relevant tickets?
Fixes #61
Issues DS-168, DS-169, DS-170