Unify licensing tagging

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Both license (American English spelling) and licence (British English spelling) are used on tens of thousands of features each. We should settle on one spelling of this key, which is significant for legal reasons.

Historically, license has been by far the more prevalent key. All the wiki’s documentation makes reference to it, and iD’s License field adds this key. Most imports, including the Atlas of Historical County Boundaries import (#418), have used license.

However, licence is currently 25% more prevalent than license due to an import of bus stops in Belgium under CC BY-SA. licence is also consistent with the OpenStreetMap convention of tagging according to British English spelling, which we’ve inherited via countless tags.