Add list of common labels

[yubiuser]

What does this PR aim to accomplish?:

Adds a list of common labels used across all Pi-hole repos

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[PromoFaux]

I don't remember where we got to with discussion around meta info like this. Does this have any functional value or does it serve as a list for manual reference?

[yubiuser]

This is just a manual list we agreed to adhere to. I think there is some kind of org-wide label system, but it includes just the common labels - each repo is free to change and add as it wants. This does not prevent us from using divergent labels which can (and did) cause issues with the workflows.