Spec link should allow multiple URLs

[cbiesinger]

Is your feature request related to a problem? Please describe.
Sometimes a feature's specification needs multiple URLs. For example, the feature affects multiple specs, or there are multiple pending PRs. However, Chromestatus currently only allows a single URL in this field.

Describe the solution you'd like
I would like to be able to specify multiple URLs for "Spec link" similar to how I can put mutiple URLs in the explainer link field.

Additional context
Add any other context or screenshots about the feature request here.

[recvfrom]

An example of where this would have been helpful, where the spec work spans the FileAPI, Fetch, and HTML specs: https://chromestatus.com/feature/5130361898795008