* 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.
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