Add "file://" protocol to stan_progress.html

[JHonaker]

Summary:

Adding the "file://" protocol to the beginning of the iframe increases compatibility with other browsers. Before, the update page would work in R Studio (which is why I think it hasn't been fixed so far) and using Edge/IE. Firefox complained about an invalid protocol, and would not display the page. After this simple change, it works as expected, and eliminates the need for "open_progress = FALSE" when using something else, like Emacs and ESS.

Intended Effect:

Increase compatibility of stan_progress.html with more browsers.

How to Verify:

Run a Stan model outside of R Studio with a non-Edge/IE default browser or open the HTML file created in the Temp directory.

Side Effects:

None

Documentation:

None needed.

Reviewer Suggestions:

Copyright and Licensing

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By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: GPLv3 (http://opensource.org/licenses/GPL-3.0)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[bgoodri]

This sounds good. I'll test it on Linux. Thanks!