Check past Agile tariff rates

[ryanw-mobile]

With the app it's now almost safe to switch tariff.
One more thing needed is to check the previous winter rate to forecast how much electricity we could consume this winter.

[ryanw-mobile]

UI:

• Since for the Agile section to be meaningful, we must have to present it using a half-hourly view, so we won't have the presentation style issues like the usage screen.
• We can simpy add a button to trigger a calendar popup to select a date, and allow to jump back to "now"
• Multiplatform Calendar widget will then be the key

Backend:

• We have to query the tariff start date, because before that day we won't be able to get any data.
• Since we now hardcode the tariff product code, this won't happen yet, but at the same time we can also get the tariff end date (if that's already ended) so we won't allow selection beyond that day.

This has to be assigned a higher priority since I am eager to know how expensive it can be during winter.