Add "hidden" flag to TradingPosition records that should be hidden in the UI

[kenkunz]

Goal

As a strategy admin, I want to be able to hide specific positions from non-admin users on frontend, in order to prevent them from being confused by erroneous data.

Background

See tradingstrategy-ai/frontend#812

As a short-term work around, positions are bing hidden in frontend by adding hiddenPositions array to the frontend strategy configuration (found in strategies.json on prod). This is not a scalable solution and is problematic b/c the strategy configuration is not defined in a single, consistent location.

Note the question under How to flag position as hidden? in the ticket description. This ticket assumes we are going with the recommendation in that section. If there is a better approach, update the ticket accordingly.

Acceptance criteria

1. Position records (found under state.portfolio.[status]_positions) include a new flags property.
2. The flags property includes a list of values, taken from an enumeration of valid position flags
3. One of the valid position flags is hidden
4. This position includes the hidden flag.