Accessibility: Bump heading in groups/single/admin.php

[mercime]

Basic rationale: see #6871

Current headings:
<h1>Group Name</h1>
<h2>Group Admins/Mods</h2>
admin.php
<h4>Privacy Options</h4> => <h2> ... </h2>
<h4>Group Invitations</h4> => <h2> ... </h2>

This also includes grouping of radio buttons to improve accessibility without using fieldsets.

[mercime]

Attached patches bump headings of the screen in Group Manage > Settings and groups two radio sets on screen by using aria-labelledby" and aria role="group" in lieu of fieldsets

• 6882.patch​ - noted above
• 6882-plus-css.patch - noted above plus adds minimum style support

Attached before & after screenshots in the following themes:

• Twenty Ten
• Twenty Sixteen
• Twenty Fifteen
• Twenty Fourteen
• Twenty Thirteen
• Twenty Twelve

[mercime]

Use fieldsets to group radio buttons

[mercime]

In 10969:

Accessibility: Use fieldsets to group radio buttons in Manage > Settings.

Fixes #6882.