Various fixes to the "Download image from URL" functionality (#4101)

* Various fixes to the "Download image from URL" functionality

This commit adds an user-agent string to the download request wich prevents some sites from
returning a 403 (forbidden) status.
Clicking the download button on the thumbnail, also showed the thumbnail because the event wasn't stopped.
The parts thumbnail didn't update after a succesfull download of the image, reloadImage was never called.

* Fixed copy and paste errors in previous commit

* Reverted back the user-agent  immitation from the previous commit

* Add a user-agent overide setting

Add a setting to allow overiding the default user-agent  that is used for downloading images and files from a URL

* Fixed a typo in the description

* Another typo fixed

* Removed cast to string to prevent using 'None' as the user-agent

* Thumbnail delete button not visible after upload

The delete button on the image thumbnails of both parts and companies
isn't showing when we upload a new image. This is fixed by always
including the element on the page, and use javascript to show and hide
the element.

InvenTree/company/templates/company/company_base.html

@@ -58,9 +58,7 @@
             {% if allow_download %}
             <button type='button' class='btn btn-outline-secondary' title="{% trans 'Download image from URL' %}" id='company-image-url'><span class='fas fa-cloud-download-alt'></span></button>
             {% endif %}
-            {% if company.image %}
             <button type='button' class='btn btn-outline-secondary' title='{% trans "Delete image" %}' id='company-image-delete'><span class='fas fa-trash-alt icon-red'></span></button>
-            {% endif %}
         </div>
     </div>
 </div>
@@ -176,6 +174,7 @@
                 showModalImage(data.image);
             });
 
+            $('#company-image-delete').show();
         } else {
             location.reload();
         }
@@ -197,6 +196,9 @@
     $('#company-image').click(function() {
         showModalImage('{{ company.image.url }}');
     });
+    {% else %}
+    $('#company-image-delete').hide();
+    {% endif %}
 
     $('#company-image-delete').click(function(event) {
         event.stopPropagation();
@@ -224,8 +226,6 @@
         );
     });
 
-    {% endif %}
-
     $("#company-image-upload").click(function(event) {
         event.stopPropagation();
         constructForm(
@@ -244,8 +244,8 @@
     });
 
     if (global_settings.INVENTREE_DOWNLOAD_FROM_URL) {
-
         $('#company-image-url').click(function() {
+            event.stopPropagation();
             constructForm(
                 '{% url "api-company-detail" company.pk %}',
                 {