MDL-80384 courseformat: Set activity icon alt text on editing mode

From a warning using WAVE:
* The title attribute value for images that lack an alt attribute value
will be presented to screen reader users. However, providing image
content in the alt attribute typically provides better accessibility,
and should be used in most cases. The title attribute will generate a
mouse hover tooltip which may or may not be desired - this tooltip will
not be presented to touch screen or keyboard users.

course/format/templates/local/content/cm/cmicon.mustache

@@ -35,7 +35,7 @@
         class="activityicon {{iconclass}}"
         data-region="activity-icon"
         data-id="{{cmid}}"
-        alt=""
+        alt="{{#showtooltip}}{{#cleanstr}} activityicon, moodle, {{{pluginname}}} {{/cleanstr}}{{/showtooltip}}"
         {{#showtooltip}}title="{{{pluginname}}}"{{/showtooltip}}
     >
 </div>