Add ARIA dialog role to Markdown editor dialogs

[gwprice]

I verified that the role is properly inserted into both the Insert Hyperlink and Insert Image dialogs and that ChromeVox responds appropriately.

@marcotuts @kevinchugh @jimabramson

[marcotuts]

this looks great. thumbs up for me. 👍

[marcotuts]

One additional note - we have a couple places in capa where we create div elements directly via javascript for the purposes of iframes. Here are most of them:

Should this be added to a backlog for a fix similar to this one? (In the meantime I've presumed yes and added to JIRA LMS)

/Users/marco/mitx_all/edx-platform/common/static/js/capa/jsme/hosted.html:
24: var iframeDiv = topDoc.createElement("div");
173: var outer = topDoc.createElement("div");

/Users/marco/mitx_all/edx-platform/common/static/js/capa/jsmolcalc/hosted.html:
24: var iframeDiv = topDoc.createElement("div");
173: var outer = topDoc.createElement("div");

/Users/marco/mitx_all/edx-platform/common/static/js/capa/protex/hosted.html:
24: var iframeDiv = topDoc.createElement("div");
173: var outer = topDoc.createElement("div");

[jimabramson]

👍