Inform user that platform is marked in maintenance mode

[ZIMkaRU]

This PR informs user that the platform is marked in maintenance mode by WS, in the framework mode

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The idea is the following:

• if an error ["error", 20060, "maintenance"] https://docs.bitfinex.com/docs/rest-general catches when fetching data from bfx api
• send emitMaintenanceTurnedOn event by WS
• also adds emitMaintenanceTurnedOff event which will be sent from the framework part in case maintenance mode is turned off
• in UI if the event is gotten show a corresponding message to inform the user that some features are unavailable

[ezewer]

looks good to me!