Highlight traffic_sign=city_limit when answering maxspeed quest

[burrscurr]

Use case
In many countries, city limit signs define maxspeed rules. When answering the maxspeed quest, the precise location of the an already tagged traffic_sign=city_limit is important to keep the data consistent. For instance, when splitting ways to answer the maxspeed quest, the split should happen at the node that's the city limit.

Proposed Solution
Show the locations of traffic_sign=city_limit on the map when the maxspeed quest is being answered.

Caveats

• traffic_sign=city_limit is very well mapped in Germany. Usage of this tag may be more rare in other countries.
• In some countries (for instance Poland) traffic_sign=city_limit does not imply maxspeed. In these countries highlighting the city limit is probably more confusing than helpful.
• One could also consider to highlight more traffic_sign values that imply or regulate maxspeed, e.g. traffic_sign=DE:274[70]. Not sure how common those variants are and whether maintaing a list of those is worth the effort.