Fix classname mismatch between upload js and templates/thumbnail.html

Fix classname mismatch between upload js and templates/thumbnail.html

Classname was changed to use newer jQuery library, but js was still
hardcoding the old classname. This resulted in the preview of the
to be uploaded file, not to be shown.

I added an html comment to the template file to warn future
modifies about the dependency. I'm not sure if html comments in
template files are considered "appropriate".

Bug: T109952
Change-Id: I098a86260453d32644ff9ab06194c13998643de9
Follow-up: 6a70cc6968a3780c0b118e07f02f5ef015e655bb