WCAG: add alt="" on school non-content icon

* https://github.com/GoogleChrome/accessibility-developer-tools/wiki/Audit-Rules#ax_text_02 says:
```
<!-- Good: image used for presentation has an empty alt value -->
<img src="line.png" alt="">
```

index.html

@@ -224,7 +224,7 @@ <h1>Results</h1>
       <div id="school-info-{{school_code}}" class="school-info">
 
         <div class="school-name">
-          <img class="icon" src="https://api.tiles.mapbox.com/v3/marker/[email protected]">
+          <img alt="" class="icon" src="https://api.tiles.mapbox.com/v3/marker/[email protected]">
           {{school_name}}
         </div>