event_index: Make button label translatable

Make the label of the button with toggles display of extern events
translatable. Put the two translated strings into template variables,
because one of it is used multiple times in the template.

mpicms/templates/events/event_index.html

@@ -2,6 +2,8 @@
 {% load event_tags i18n %}
 
 {% block content %}
+    {% translate "hide external events" as txt_hide_external_events %}
+    {% translate "show external events" as txt_show_external_events %}
     <div class="section">
 
         <h2 class="title is-2">{{ page.title }}</h2>
@@ -15,7 +17,7 @@ <h2 class="title is-2">{{ page.title }}</h2>
             <div class="fc-center">
             </div>
             <div class="fc-right">
-                <a id="switch_external" class="button">hide external events</a>
+                <a id="switch_external" class="button">{{ txt_hide_external_events }}</a>
             </div>
         </div>
     </div>
@@ -46,11 +48,11 @@ <h2 class="title is-2">{{ page.title }}</h2>
                 if (externalEventsShown) {
                     calendar.getEventSources()[1].remove()
                     externalEventsShown = 0
-                    this.innerHTML="show external events"
+                    this.innerHTML="{{ txt_show_external_events }}"
                 } else {
                     calendar.addEventSource(event_source_external)
                     externalEventsShown = 1
-                    this.innerHTML="hide external events"
+                    this.innerHTML="{{ txt_hide_external_events }}"
                 }
             });