Add new volunteer role "parkwalker"

[pwhettlock]

In Oct 2022, parkrun introduced the new volunteer role of "parkwalker".

Athlete ID 519096 can be used to test this, and Halifax parkrun has regular parkwalkers on their rota.

This will require a new vest-style badge, which I've mocked-up.

Maybe a good time to also refresh the RD vest to avoid confusion - not 100% sure if the bottom should now be blue or black?

[pwhettlock]

HQ have confirmed parkwalkers are here to stay!

[pwhettlock]

I can see @sneaker-net has made a pull request for the code files: https://github.com/fraz3alpha/running-challenges/pull/364

Assuming they're OK, just need the images sorting(?)

[fraz3alpha]

https://github.com/fraz3alpha/running-challenges/pull/381 has the changes to add it, and it is available from https://github.com/fraz3alpha/running-challenges/actions/runs/4758726514

It also comes with a colour refresh for all the hi-viz