UI Accessibility Improvements: Group membership entity table lacks column header for fourth column

Problem:
The definition is read as four columns but the fourth column (containing the Actions dropdowns) has no column header to let the screen reader user know what is in that column.

Solution:
Add column heading text to the <th> statement as shown below
Note that this text can be hidden from the visual user using CSS (See page 7 for the proper way to hide text with CSS)
<th class="sorted">Entity name</th>
<th data-hide="phone">Membership</th>
<th style="width:100px;" class=”hidden”>Choose action</th>