Ensure header cells are not blank

Blank cells should not be headers for data cells.  Change the upper left
<th>, which is always blank, to a <td>.

SOL-1996

poll/public/css/poll.css

@@ -166,6 +166,7 @@ li.poll-result .poll-image {
     background: none;
 }
 
+.survey-table thead tr td,
 .survey-table thead tr th {
     font-weight: bold;
     font-size: .9rem;

poll/public/css/themes/lms.css

@@ -13,6 +13,7 @@
     vertical-align: middle;
 }
 
+.themed-xblock.poll-block .survey-table thead tr td,
 .themed-xblock.poll-block .survey-table thead tr th {
     font-size: 1em;
 }

poll/public/handlebars/survey_results.handlebars

@@ -3,7 +3,7 @@
     <table class="survey-table poll-results">
         <thead>
             <tr>
-                <th></th>
+                <td></td>
                 {{#each answers}}
                     <th class="survey-answer">{{{this}}}</th>
                 {{/each}}

poll/public/html/survey.html

@@ -6,7 +6,7 @@
             <table class="survey-table">
                 <thead>
                   <tr>
-                      <th></th>
+                      <td></td>
                       {% for answer, label in answers %}
                         <th class="survey-answer">{{label}}</th>
                       {% endfor %}