Rework finding of preview lectures.

This time, we only grab preview lectures and nothing else.
Fix breakage of pages pointing to wikipedia.

The solution that we take here is still suboptimal, in the sense that we
report a link to external sources as `None`, but we are on the right path
for further processing.
Revert (with modifications) "Remove unused code for hidden videos.".

This reverts commit 5ca7dbb9, since we still have uses that want to grab
those hidden videos by the University of Washington.

This closes #95.