Multiple file uploads

[g105b]

In this PR:

• HTML file input has new multiple attribute
• PHP loops over the files as a collection, rather than assuming a single upload
• List of uploads are listed to the user
• User can remove one or all of the uploads at will
• Drag and drop as a bonus extra

[g105b]

These commits were just merged in the other PR - just checked locally and we have all expected functionality.