Add diff option to deploy.py

[manics]

I worked on this in #2698 but decided to make it into a seperate PR so we can discuss whether it's useful.

Adds a --diff option that uses kubectl diff (builtin) and helm diff (requires https://github.com/databus23/helm-diff) which is useful for local development.

It's not ideal since if there's a diff it shows the whole file instead of just the changed region, and CHP/hub will always show a difference because helm diff always generates a random token instead of fetching the CHP token.

[minrk]

This is great!

If it's useful for reference, I've used this script to check out helm-diffs for mybinder.org for some time.

specifically piping helm-diff through | grep -C 1 -v '^[[:space:]]' to limit the output to nearby changes.

[manics]

I've belatedly realised helm diff includes a --context argument!