Fix: CEM work session

[Flix6x]

Follow-up to #187.

• Fix handshake handler now sending back two responses against a queue that is periodically flushed (every 0.3 seconds)
• Docker-compose stack skips running toy tutorial every time -> this is actually just for devs at the moment, because the toy account is actually needed for the CEM to run; specifically, the CEM uses the toy user credentials
• Ported the FRBC.FillLevelTargetProfile and FRBC.UsageForecast to the FRBCSimple handler.
• Test CEM now configures its own data dashboard in FlexMeasures
• Test CEM now polls FlexMeasures much faster, which is more suitable for simulations.
• A few minor fixes to the SimpleFRBC handler to get to the point that FRBCInstructions are sent back.
• Ported the FRBC.LeakageBehaviour to the FRBCSimple handler.
• Support wrapped S2 messages in an envelope together with metadata containing the simulation time.
• Made the start of the schedule equal to the most recent storage status