Replace both C3 and Dygraph with ChartJS

This is almost iso-functional: we lose the subchart, but considered how buggy it was (especially the "intent" settings), was as straighforward to just implement a range selector + predefined period ranges (for the in-out chart).

About that, there's a CSS issue I don't quite understand; the buttons-group in the Form.Input seems to override the maximum length. That's ok in desktop mode and in this screenshot, but on mobile this means the button may escape the viewport to the right. @nicofrand would you have an idea on how to fix this properly? I'd like to add moar buttons (last year, current year) in the future, so as to unify the date selectors everywhere in the app (including in the reports list).

Fixes #1033 (closed).