* feat: adds VerticalBlockChildrenLoaded filter call
This introduces the VerticalBlockChildrenLoaded filter that is run after
all the child blocks are fetched before rendering a student or the
public view. This will allow modifying the contents of the VerticalBlock
before presenting it to the students.
- Moving xmodule folder to root as we're dissolving sub-projects of common folder in edx-platform
- More info: https://openedx.atlassian.net/browse/BOM-2579
- -e common/lib/xmodule has been removed from the requirements as xmodule has itself become the part of edx-platform and not being installed through requirements
- The test files common/lib/xmodule/test_files/ have been removed as they are not being used anymore
