Add filesize check when uploading a new file
Fix error 500 if the uploaded background was not an unsplash one
Add upload background setting to info endpoint
Add config docs
Fix lint
Return list background type if it was uploaded
Add file upload
Add docs
Save uploaded backgrounds as file
Add background upload handler
Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/582
