i18n fixes (#3965)

* i18n fixes

Use i18n for search placeholder
Add i18n usage for Surcharge and Range abstract models where it was
missing

* Add trans to aria-label

* Add missing verbose names in migrations

---------

Co-authored-by: Viggodevries <viggo@highbiza.nl>

src/oscar/apps/offer/abstract_models.py

     slug = fields.AutoSlugField(
         _("Slug"), max_length=128, unique=True, populate_from="name")
 
-    description = models.TextField(blank=True)
+    description = models.TextField(_("Description"), blank=True)
 
     # Whether this range is public
     is_public = models.BooleanField(

src/oscar/apps/offer/migrations/0001_initial.py

                 ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                 ('name', models.CharField(unique=True, max_length=128, verbose_name='Name')),
                 ('slug', oscar.models.fields.autoslugfield.AutoSlugField(populate_from='name', unique=True, verbose_name='Slug', max_length=128, editable=False, blank=True)),
-                ('description', models.TextField(blank=True)),
+                ('description', models.TextField(verbose_name="Description", blank=True)),
                 ('is_public', models.BooleanField(default=False, verbose_name='Is public?', help_text='Public ranges have a customer-facing page')),
                 ('includes_all_products', models.BooleanField(default=False, verbose_name='Includes all products?')),
                 ('proxy_class', oscar.models.fields.NullCharField(unique=True, verbose_name='Custom class', default=None, max_length=255)),

src/oscar/apps/order/abstract_models.py

         abstract = True
         app_label = 'order'
         ordering = ['pk']
+        verbose_name = _("Surcharge")
+        verbose_name_plural = _("Surcharges")

src/oscar/apps/order/migrations/0009_surcharge.py

             ],
             options={
                 'abstract': False,
-                'ordering': ['pk']
+                'ordering': ['pk'],
+                'verbose_name': 'Surcharge',
+                'verbose_name_plural': 'Surcharges'
             },
         ),
     ]

src/oscar/templates/oscar/partials/search.html

 {% load i18n %}
 
 <form class="form-inline my-2 my-lg-0" method="GET" action="{% url 'search:search' %}">
-    <input class="form-control mr-sm-2" name="q" placeholder="Search" aria-label="Search">
+    <input class="form-control mr-sm-2" name="q" placeholder="{% trans 'Search' %}" aria-label="{% trans 'Search' %}">
     <button class="btn btn btn-secondary my-2 my-sm-0" type="submit">{% trans "Search" %}</button>
 </form>