Make update-khan-site.sh work with new combine.py script

Also update the sed regex to prevent it from emitting two consecutive slashes.
Regenerate khan-site.css

Note that the last version seems to have a lot of duplicate css for
some reason.
Remove weird creation of empty file
Update khan-site.css yet again

Rerunning it gives me a huge diff, so I suspect the last one was generated
from an out of date website repo or something
Merge pull request #36829 from Khan/update-qtip

Tweaks from review K91485