The Randomized Content Block XBlock only randomizes the selection of
the children blocks and has unpredictable randomization of
the order of the selected child blocks due to the usage of sets, which
are unordered, for storing the selected blocks. This becomes apparent
when all the available child blocks in a library are chosen for a
Randomized Content Block, to randomize just the order of the child
blocks and not just the selection of the blocks. The order of the
selected blocks ends up being similar for multiple learners.

This change modifies the XBlock to store the selected child blocks in
a list, instead of a set, after randomly shuffling them.

A new block structure transformer, ContentLibraryOrderTransformer has
been added to transform the order of the selected children XBlocks to
match the order of the selections made in the
ContentLibraryTransformer. This ensures that same correct randomized order
of blocks is also returned by the course blocks API.