feat(helm): add priorityClassName attribute

[jperville]

This PR adds an optional priorityClassName attribute to the helm chart values.yml
The default value is an empty string to be compatible with the existing behavior.

Tests:

# with the default/empty priorityClassName:
helm template my-mailcrab . | grep --color priorityClassName -C2

# with a custom priorityClassName
helm template my-mailcrab . --set priorityClassName=xyzzy | grep --color priorityClassName -C2

[jperville]

@marlonbaeten would it be possible to review this simple PR for mailcrab helm chart? Thanks a lot.