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/part/templates/part/part_base.html

@@ -391,6 +391,8 @@
     $('#part-thumb').click(function() {
         showModalImage('{{ part.image.url }}');
     });
+    {% else %}
+    $('#part-image-delete').hide();
     {% endif %}
 
     function reloadImage(data) {
@@ -403,6 +405,7 @@
                 showModalImage(data.image);
             });
 
+            $("#part-image-delete").show();
         } else {
             // Otherwise, reload the page
             location.reload();
@@ -586,8 +589,8 @@
     {% if roles.part.change %}
 
     if (global_settings.INVENTREE_DOWNLOAD_FROM_URL) {
-
         $("#part-image-url").click(function() {
+            event.stopPropagation();
             constructForm(
                 '{% url "api-part-detail" part.pk %}',
                 {
@@ -596,7 +599,9 @@
                     fields: {
                         remote_image: {},
                     },
-                    onSuccess: onSelectImage,
+                    onSuccess: function(data) {
+                        reloadImage(data);
+                    },
                 }
             );
         });

InvenTree/part/templates/part/part_thumb.html

@@ -13,9 +13,7 @@
             {% if allow_download %}
             <button type='button' class='btn btn-outline-secondary' title="{% trans 'Download image from URL' %}" id='part-image-url'><span class='fas fa-cloud-download-alt'></span></button>
             {% endif %}
-            {% if part.image %}
             <button type='button' class='btn btn-outline-secondary' title='{% trans "Delete image" %}' id='part-image-delete'><span class='fas fa-trash-alt icon-red'></span></button>
-            {% endif %}
         </div>
     </div>
     {% endif %}