label steep hills in instructions

[graue]

Definitions:

Steep: 8%
Very steep: 14%

Based on the maximum grade measured over any 500' length

Fixes #173

Future work:

• Ultimately remove the grade percentages. I'm erring on the side of including more info as we test this feature out and assess whether my definitions and 500' threshold work well.
• "Steep hill" should not be colored green. Maybe a gray pill.
• Indicate steep hills visually on the map.
• Show elevation profiles for legs (or entire routes).