adds and alt attribute to the loading/spinner graphic

common/lib/capa/capa/templates/formulaequationinput.html

@@ -25,7 +25,7 @@
 
       <div id="input_${id}_preview" class="equation">
         \[\]
-        <img src="${STATIC_URL}images/spinner.gif" class="loading"/>
+        <img src="${STATIC_URL}images/spinner.gif" class="loading" alt="Loading"/>
       </div>
     <p id="answer_${id}" class="answer"></p>
   </div>

common/lib/capa/capa/templates/imageinput.html

@@ -22,6 +22,7 @@
         src="${STATIC_URL}images/green-pointer.png"
         id="cross_${id}"
         style="position: absolute; top: ${gy}px; left: ${gx}px;"
+        alt="Selection indicator"
       />
     </div>
     <div