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/common/models.py

@@ -875,6 +875,12 @@ class InvenTreeSetting(BaseInvenTreeSetting):
             ]
         },
 
+        'INVENTREE_DOWNLOAD_FROM_URL_USER_AGENT': {
+            'name': _('User-agent used to download from URL'),
+            'description': _('Allow to override the user-agent used to download images and files from external URL (leave blank for the default)'),
+            'default': '',
+        },
+
         'INVENTREE_REQUIRE_CONFIRM': {
             'name': _('Require confirm'),
             'description': _('Require explicit user confirmation for certain action.'),