Tariff screen: add rate checker to the tariff detail screen

[ryanw-mobile]

This is a low-priority task, as it is purely for public demo purposes.
I would not have an option to switch to most of the tariffs except the flexible or the fixed ones.

We are now hiding the standard unit rate for variable tariffs.
This makes all variable tariff detail screens virtually meaningless.

When we have confirmed which chart library to use (in order not to waste time rewriting the code), we can put a chart on every detail screen so it can show the current/past variable rates for each tariff.

This is literally a trimmed-down version of the Agile screen (without the countdown and long list of breakdowns), just a chart, some buttons to select the date range, and a few lines with some interesting figures.