feat: origin-destination flows in the dashboard

[balacmi]

• This is task 27
• Clicking on an origin zone (i.e., canton, municipality), one can see in the dashboard the flow between that and all the other zones
• Also clicking on the zone (this time destination) one can see in the dashboard the flow coming into that zone from other zones
• Filters should include: mode, trip purpose, and time period configurable with a slider (15min granularity)

[balacmi]

@MichelleChaoChen could we add a checkbox for filters, so that multiple modes or purposes can be selected?