Drag and drop blocks does not work with subpage values and custom regions

This primarily helps to bring in support for Flexpage with drag and drop block support. But, there is nothing unique happening here. For example: I believe this would allow drag and drop blocks to work on the My Moodle page with the custom center block region of "content" (may need some extra dom around the content block region).

So, changes include:

• Passing subpage value to lib/ajax/blocks.php
• Don't assume that every region starts with "side-"