Add url dialog for to allow copying for non-github element repos

[shawncplus]

Currently if an element is coming from a bower url which isn't in the github user/repo format the github links break. This update adds a dialog to allow the user to copy the url if it's not a github link.

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[shawncplus]

I signed it!

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We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.

[shawncplus]

I signed it!

[googlebot]

CLAs look good, thanks!

[shawncplus]

If I had to do this a different way I'd make an element-source element since this _githubLink() happens in app/elements/pages/page-element.html as well