Add disk monitoring

[iandesj]

This is continuation of what #232 is trying to enhance!

Additionally, this PR:

• Fixes computing the disk warning state
• Propagates all resources in warning, not just memory
• Tweaks the react condition to look at all warnings

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[iandesj]

@krassowski Would you be able to add the correct label(s)?

[iandesj]

@krassowski (and other contrib's) what would help get this across the line?

[iandesj]

My project has been using these changes in production for nearly a month now.