Implement advanced features of currency graph for dashboard

[slaweet]

Expected behaviour

A subtask of #59.

Missing parts are:

• The graph is using hard-coded data because the relevant part of Explorer API is broken and being fixed at the moment.
• Switch to different time spans (7d, 1m, 3m, ...)
• Switch to LSK/USD and LSK/EUR
• Styles that are not supported by chart.js (or other chart libs I checked). We can fork chart.js to add those when we have more time or think of some hack to work around it:
  • tooltip: shadow, background transparency gradient and bottom padding smaller than top padding,
  • red-orange gradient color of a hovered point,
  • vertical lines (the problem is to have them between values, with gradient and side shadow on hover)

https://drive.google.com/drive/u/2/folders/1B1wiyNp-ERR5ETVBpLhVEG7L5598oe7O

Actual behaviour

There is a basic graph.