Added conditional content-security-policy header

[cgorman]

If any of the URLs start with https then we add the
Content-Security-Policy : upgrade-insecure-requests header to
DICOMwebClient requests. Addresses #159

[cgorman]

Let me know if there's a better way of doing this, thanks!

[github-actions]

Visit the preview URL for this PR (updated for commit c48584a):

https://idc-external-006--pr160-dev-cg-upgrade-insec-409byuig.web.app

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[igoroctaviano]

I think a single JSON.stringify(serverSettings).includes('https') would be enough to set this header. I'm not sure if this should be here though, maybe we need to add this change to dicomweb-client?