Fixing template variables (#2879)

* subtitle is not always defined.

* model_name_plural does not exist. Should be verbose_name_plural.

wagtail/contrib/modeladmin/templates/modeladmin/includes/search_form.html

@@ -7,7 +7,7 @@
                 <label for="id_q" class="visuallyhidden">{% trans 'Search for' %}</label>
                 <div class="field-content">
                     <div class="input icon-search ">
-                        <input id="id_q" name="{{ search_var }}" value="{{ view.query }}" placeholder="{% blocktrans with view.model_name_plural|lower as name %}Search {{ name }}{% endblocktrans %}" type="text">
+                        <input id="id_q" name="{{ search_var }}" value="{{ view.query }}" placeholder="{% blocktrans with view.verbose_name_plural|lower as name %}Search {{ name }}{% endblocktrans %}" type="text">
                         <span></span>
                     </div>
                 </div>

wagtail/wagtailadmin/templates/wagtailadmin/shared/header.html

@@ -21,7 +21,7 @@
     <div class="row">
         <div class="left">
             <div class="col">
-                <h1 class="icon icon-{{ icon }}">{{ title }} <span>{{ subtitle }}</span></h1>
+                <h1 class="icon icon-{{ icon }}">{{ title }}{% if subtitle %} <span>{{ subtitle }}</span>{% endif %}</h1>
             </div>
             {% if search_url %}
                 <form class="col search-form" action="{% url search_url %}{% if query_parameters %}?{{ query_parameters }}{% endif %}" method="get">
@@ -38,7 +38,7 @@
             {% usage_count_enabled as uc_enabled %}
             {% if uc_enabled and usage_object %}
                 <div class="usagecount">
-                    <a href="{{ usage_object.usage_url }}">{% blocktrans count useage_count=usage_object.get_usage.count %}Used {{ useage_count  }} time{% plural %}Used {{ useage_count }} times{% endblocktrans %}</a>
+                    <a href="{{ usage_object.usage_url }}">{% blocktrans count useage_count=usage_object.get_usage.count %}Used {{ useage_count }} time{% plural %}Used {{ useage_count }} times{% endblocktrans %}</a>
                 </div>
             {% endif %}
             {% if add_link %}