file.html: Add an aria-label to the coverage column

The column with the coverage info can be:

  no coverage (displays nothing, normal background)
  0 (displays nothing, color-coded)
  number (color-coded)

For screen reader users, we make it be like these, respectively:

  no coverage (says "no coverage" in the aria_label)
  0 (says "0")
  number (as usual)

src/templates/file.html

@@ -11,14 +11,17 @@
             {%- set highlight = "success" -%}
             {%- set highlight_light = "success-light" -%}
             {%- set count = item.1 -%}
+            {%- set aria_label = count -%}
         {%- elif item.1 < 0 -%}
             {% set highlight = "white" -%}
             {% set highlight_light = "white" -%}
             {% set count = "" -%}
+            {%- set aria_label = "no coverage" -%}
         {%- else -%}
             {%- set highlight = "danger" -%}
             {%- set highlight_light = "danger-light" -%}
             {%- set count = "" -%}
+            {%- set aria_label = "0" -%}
         {%- endif -%}
         <div class="columns p-0 m-0" role="row">
             <div
@@ -29,7 +32,7 @@
             </div>
             <div
                 class="column is-1 is-narrow p-0 has-text-centered has-text-{{ highlight_light }} has-background-{{ highlight }}"
-                role="cell">
+                role="cell" aria-label="{{ aria_label }}">
                 {{ count }}
             </div>
             <div class="column has-background-{{ highlight_light }} p-0"