Add crash markers with popups to UI

[lydiascarf]

Overview

This PR adds crash data to the client

Notes

I had to make a couple of compromises

1. I moved the legends to the bottom left so they wouldn't interfere with the layer picker
2. I didn't add a new legend and instead added a hint to the other active transportation markers

Demo

Testing Instructions

• Run scripts/update so you can get the new leaflet dependency
• Start up the server
• Go to a place where you have an analysis job, e.g. http://localhost:9301/#/places/some_analysis_job_uuid
• Open the layer picker and select crashes
• You should be able to see crash markers and clustered markers on the map
• You should be able to see popups when you click on the markers that show year, fatalities, and an optional hint if the crash is an active transportation crash of any kind, and another optional hint if it is either an active transportation crash or a bike crash with multiple fatalities

Checklist

• [x] Add entry to CHANGELOG.md

Resolves #921 Resolves #922 Resolves #923

[lydiascarf]

@KlaasH this is functionally ready to review but just so you know Philipp and I are going to tweak some marker and font sizing in the morning

[lydiascarf]

@KlaasH one copy edit pending but no CSS changes left