Curb validator fixes

[tannerwuster]

Fixes following #1583

• Validator didn’t flag missing curb on a crossing where the curb on one side is already mapped
  Old
  New
• Because the crossings are connected to cycleways rather than sidewalks, no issues are flagged within the validator.
  Old
  New
• New way that is created always takes sidewalk tag even though its not always a sidewalk
  You can test this on the above code where the extension way is a cycle way not a sidewalk like the old code use to work.
• Add a link to the kerb tagging guidelines in the OSM wiki when the user clicks the 'i'?

closes #1619