Infer missing speed limits

[dabreegster]

The speed limit layer added in #169 is missing data, as described in the PR, discussed in #177, and visible clearly on any map.

I was considering integrating with https://github.com/westnordost/osm-legal-default-speeds to infer missing values, but currently, the rules for GB, which seem to match https://www.gov.uk/speed-limits, do not seem to take effect. I plugged https://www.openstreetmap.org/way/74150037 as an example highway=residential in at https://westnordost.github.io/osm-legal-default-speeds/#tags=highway%3Dresidential&cc=GB, but it didn't match any rules.

Opening this issue to track all possible solutions. I would be in favor in the UI of having an option to only show explicitly-tagged-in-OSM speeds, but disabling and de-emphasizing by default.

[robinlovelace-ate]

Sounds good, I think residential being 30 or less is a good starting point and thumbs up to representing that in the UX.

[dabreegster]

We have more thorough data from OS now