Validator: Add lower issue severity for 'suggestions'

[bhousel]

Spun off from #1619..

We received feedback that the missing kerb issues should really be a lower level severity than a "warning" - it's more of a "suggestion". The missing kerbs are not a routing problem, but having this information tagged is helpful to some consumers.

So today I added a lower level of severity called a "suggestion".
👉 Fun Fact! We've actually wanted to do this for years - see openstreetmap/iD#6101

So now our validator supports 3 levels of severity:

Errors are important enough that the user needs to fix them before they can upload their changes.

Warnings are actual issues with the map data - they should be fixed, but the user can still ignore them.

Suggestions are for issues that really are less severe than warnings. We might move some of our existing warnings down to this lower level. We will make it so that users can disable them completely if they don't want the extra hints.

Also, I've made it so that the counts of these things now correctly appear down in the Validator Status section of the footer.