Add Bouldering grade filter popover

[vnugent]

Crag Finder story #178

Use case: as a boulderer I want to find bouldering areas with problems within my grade.

Tips:

The V grade component should be very similar to YDS filter and Radius filter.

• YDS: https://github.com/OpenBeta/open-tacos/blob/develop/src/components/YDSFilter.tsx
• Radius: https://github.com/OpenBeta/open-tacos/blob/develop/src/components/RadiusFilter.tsx

[vnugent]

@clintonlunn FYI this could be good one in case you're interested in a UI task