Make MathJax show errors for bad TeX but only in the problem editor. (another alternate for #2837 and #2838)

[drgrice1]

There have been requests to either remove this extension or at least make it so that those editing problems do not have it loaded, as it makes it easier to determine what is wrong with TeX in a problem.

This pull request makes it so that these errors are only shown in the PG problem editor. Actually they are shown any time the renderRPC endpoint is used and the showMathJaxErrors parameter is set to a true value, but the only time that webwork2 now does this is in the problem editor.

Of #2837, #2838, and this pull request, I think this is the way to go.

This still includes thhe change from the webwork_url to the webwork_js_config method in the WeBWorK::ContentGenerator module. There is also a webwork_url method in the Mojolicious::WeBWorK module that is already available for all controller modules (since it is a Mojolicious helper method), and having this other one overrides that one and it is confusing to have both that return almost the same value. The only difference is that WeBWorK::ContentGenerator method called the location helper which returns the empty string if the root URL is '/', and the webwork_url helper returns '/' in that case. I don't know what I was thinking creating the WeBWorK::ContentGenerator method which was really just an alies for the location helper method anyway.

[somiaj]

Test works fine, the error is only shown in the problem editor. Otherwise the raw tex is shown in yellow, so an error is known just not what it is.

[drgrice1]

Note that this does not show TeX errors if you are using the problem editor and render in a new window. I think it is not worth it to add that.