Improve new enroll kb user experience

The main content area of each panel was in the normal tab order, though
there is no visual indication, and its not a functional stop. This
commit removes it, though still lets the modal set focus on the main
content any time the user switches panels. This lets the screen reader
user get informed of the new context w/o interfering with the kb only
user.

fixes CNVS-35469

test plan:
  - go to the course people page
  - click on +People
  - the modal opens and focus is on the close button
  - press TAB
  > expected result: focus moves to the radio button
  - now do the same thing with a screen reader, and instead of
    pressing tab, use the SR's keyboard command to start reading.
  > expected result: you are told the purpose of the panel
  - enter some stuff, navigate to the Next button and press it
  > expected result: focus is on the panel's content area and you are
    read the purpose of the panel

there are no specs because this is strictly an interactive a11y thing.

Change-Id: I9ab23daa2240e290449622c369f2bbb1a5facc78
Reviewed-on: https://gerrit.instructure.com/104555
Tested-by: Jenkins
Reviewed-by: Felix Milea-Ciobanu <fmileaciobanu@instructure.com>
QA-Review: Dan Sasaki
Product-Review: Ed Schiebel <eschiebel@instructure.com>

Author: eschiebel

app/jsx/add_people/components/add_people.jsx

@@ -292,7 +292,7 @@ define([
           <ModalBody>
             <div
               className="addpeople"
-              tabIndex="0"
+              tabIndex="-1"
               ref={(elem) => { this.content = elem }}
               aria-label={panelLabel}
               aria-describedby="addpeople_panelDescription"