Previously, the built-in XBlocks (problem, video, etc) could be parsed
from pointer tag syntax, but externally-defined XBlocks (drag-and-drop,
ORA, etc.) could only be parsed from inline syntax. This is because the
built-in blocks have special parsing logic, defined in XmlMixin,
which is not available to external blocks.
This PR shifts the pointer tag parsing "up a level" such that the parent
blocks parse the pointer tag, regardless of whether the child is built-in
or external:
* vertical (aka unit)
* split_test (aka content experiment)
* itembank (aka problem bank)
* library_content (aka randomized legacy library)
The following parent blocks still lack support for external pointer-tag children;
we will fix this in a follow-up PR:
* randomize
* all externally defined container blocks
Part of: https://github.com/openedx/XBlock/issues/823
d382721cf8