Add the ability to upload attachments when copying mixed content from one server to another

Issue Summary

This is reproducible on Data Center: yes
Currently, Confluence does not upload images when copying/pasting content from a page on an instance to another instance. This results in the images being inserted as external images with their link pointing to the source instance where they were copied from.

Steps to Reproduce

1. For this test, two Confluence instances are needed
2. Go to instance 1, open a page (with mixed content, attachment images and text) >> click edit >> Select all content on the page, then copy
3. Go to instance 2, open a page >> create a page >> paste content >> Update
4. View the storage format of the page and notice that images were inserted as external images rather than attachments 

Expected Results

When the content is pasted from another Confluence instance, the images should be uploaded and then inserted as attachments

Actual Results

When the content is pasted from another Confluence instance, the images are inserted as external images loaded from the source instance

Workaround

Option1 - single page

• Copy/paste the content from the source to the target instance
• Download the images to your machine
• Upload images to the target page manually
• Replace images to insert the ones from the attachments

Option2 - Multiple pages

• Create a new space on the source instance with a unique space key (doesn't exist on the target server)
• Copy all pages that you want to transfer to that new space
• Export the new space to XML
• Restore on the target server
• Move the pages from the restored space to their target location