fix: the get_completion JS handler should not ignore FBE blocks

Because xblock handlers normally get a block tree that already has
inaccessible blocks stripped, they don't see FBE gated blocks at
all. So the get_completion handler would return True for FBE units
incorrectly. Leading to a visual bug as an audit user went through
their units in the courseware.

In order to let the handler know about the full tree, I've added a
new attribute you can set on your xblock handlers:
my_handler.will_recheck_access = True

This will tell the top-level handler code get the full tree for you.

As part of this, I've also changed the sequence xblock handler's
into proper xblock handlers (not old-style xmodule handlers).
This changes their URLs slightly. I've kept the old URLs for now
as well, but they'll be removed after Maple.

AA-409