Use url field for urls about db HOT 4 CLOSED

in PR #25 @fabrice102 said that old styles do not use the url field and thus they are unsure how to cope with this in an backward compatible way.
Would it be a solution to duplicate the url and have them in note and url fields?

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I have no strong opinion either way. I'd guess url is now meant to be the canonical way (?)

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The issue with keeping both url and note is that the url appears twice in some cases.
The issue with having only url is that classical BibTeX styles would not work anymore: http://www.tex.ac.uk/FAQ-citeURL.html

Maybe, we should still switch to url.

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We decided to use the url field.

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