Fix nested sequence display in Courseware MFE
Fix nested sequence display in Courseware MFE

Check to make sure that "activate_block_id" is actually a Unit
(VerticalBlock) before using it to build a link to the Courseware
MFE. We were previously sending sequence block keys here, causing
the MFE to render a whole sequence where only a unit was desired.

Fixes TNL-7115

cpennington/edx-platform,cpennington/edx-platform,stvstnfrd/edx-platform,eduNEXT/edunext-platform,eduNEXT/edunext-platform,angelapper/edx-platform,msegado/edx-platform,appsembler/edx-platform,arbrandes/edx-platform,eduNEXT/edunext-platform,arbrandes/edx-platform,EDUlib/edx-platform,edx-solutions/edx-platform,appsembler/edx-platform,mitocw/edx-platform,msegado/edx-platform,arbrandes/edx-platform,msegado/edx-platform,angelapper/edx-platform,edx-solutions/edx-platform,mitocw/edx-platform,eduNEXT/edx-platform,eduNEXT/edunext-platform,EDUlib/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,stvstnfrd/edx-platform,edx-solutions/edx-platform,arbrandes/edx-platform,msegado/edx-platform,edx/edx-platform,edx/edx-platform,msegado/edx-platform,cpennington/edx-platform,EDUlib/edx-platform,EDUlib/edx-platform,edx/edx-platform,mitocw/edx-platform,stvstnfrd/edx-platform,angelapper/edx-platform,edx/edx-platform,mitocw/edx-platform,stvstnfrd/edx-platform,eduNEXT/edx-platform,edx-solutions/edx-platform,appsembler/edx-platform,eduNEXT/edx-platform,appsembler/edx-platform,cpennington/edx-platform