Bug 69331: Represent <section> tags as invisible meta tags

This improves the rendering in wikisource for now by not emitting those tags
as plain text any more. It is not the final solution to the LST problem, which
is discussed at https://www.mediawiki.org/wiki/Parsoid/HTML_based_LST.

Change-Id: If30825ce54e28ceb0f02d1f751849a2582b61e6e
Merge "Bug 69331: Represent <section> tags as invisible meta tags"