Changed responsive images behavior, updated upload and transfer mechanisms

• Changed plugin responsive images default mechanism to leverage default WordPress srcset behavior improved with our smart image quality and auto format features.
• Returned Uploadcare upload button to a default upload option.
• Added an option to transfer files to Uploadcare storage post by post.
• Created console utility to transfer all files at once (https://github.com/uploadcare/uploadcare-wordpress-transfer/).
• Minor bug fixes and improvements.