Support O(100) cities

We've been adding a few other cities slowly and with considerable effort. What would it take to maintain a few hundred?

## Downloading optional content

- a UI for selecting extra maps to download
- persisting game `Options` between game versions
- Adapt the downloading functions from `updater` or `importer`
- Glue to `ctx.loading_screen`
- Figure out how to make web-compatible

## Storing the maps

- Currently just the latest (github master) version in Dropbox
- Likely need to keep 3 versions: github master, the latest binary release, and the last binary release (#195)
- Dropbox sucks, probably switch to S3
- There's a unique URL for each file shared from Dropbox, so `data/MANIFEST.txt` has to store the mapping
- the Dropbox daemon crashes constantly when uploading lots of new files
- want one URL, aka, https://abstreet.s3.us-west-2.amazonaws.com/latest/system/maps/southbank.bin
- ideally use Unix rsync and ditch most of `updater --upload`
- Namespace `data/system/maps/` better

## Map config

- Could just start from https://download.bbbike.org/osm/bbbike/
- People can contribute better bounding polygons
- Move to text config, instead of all the code in `importer`
- For curated cities like Seattle, keep the code of course, but for the majority of the "second tier" cities, just have basic config

## Maintaining the maps

- My laptop has a core or two, but probably it's a bit intense to regularly run the importer for hundreds of maps
- Also I'm the only person who can upload new data to Dropbox currently, doesn't scale for potential team growth
- Ideally, a CI-like server would do the work
- But it's weird to make this asynchronous; when a commit with a binary incompatible change goes in, the data should be uploaded already
- Maybe locally, a dev can send the server a command to regen everything, stage it somewhere. Then they send another command to "commit" the data, and then push their change at the same time