MDL-77898 calendar: Add iconclass for upcoming_mini template

The icon's iconclass context data adds additional CSS class(es) to
calendar event icons to better control how the event icon is displayed.
e.g. without filtering for activity events that don't hae monologo
versions of their icons.

calendar/templates/upcoming_mini.mustache

@@ -47,7 +47,7 @@
             <div class="activityiconcontainer small {{{purpose}}} courseicon mr-3">
                 {{#icon}}
                 {{#iconurl}}
-                    <img alt="{{alttext}}" title="{{alttext}}" src="{{{ iconurl }}}" class="icon ">
+                    <img alt="{{alttext}}" title="{{alttext}}" src="{{{ iconurl }}}" class="icon {{iconclass}}">
                 {{/iconurl}}
                 {{^iconurl}}
                     {{#pix}} {{key}}, {{component}}, {{alttext}} {{/pix}}