<script type="text/template" id="xblock-progress-template">
    <% if (completed === "True") {{ %>
        <img src="{completed}">
    <% }} else {{ %>
        (Not completed)
    <% }} %>
</script>

<script type="text/template" id="xblock-progress-template">
    <% if (completed === "True") {{ %>
        <img src="{completed}" alt="Completed">
    <% }} else {{ %>
        (Not completed)
    <% }} %>
</script>

Add `alt` to completion `<img>` for accessibility
html-css-review: Add `alt` to completion `<img>` for accessibility
