Use CORS in context popup

@oterral noticed we were still using CORS in context popup directive and help service.

This PR changes that for homogeneity sake.

We have one last request in jsonp in the helpService on google fusiontables, but we can't change that since it is performed over https context.

I added a commit to perform a more robust check on relative URLs.
`if (/^https?:\//\/.test(requestUrl))`