Bug 51090: Render invalid wikilinks as plain text

Some tests are failing the html2html, wt2wt and html2wt modes because of
<nowiki> wrapping. These will be fixed after updating nowiki escaping
code in the serailizer (in a later patch).

Change-Id: I04507a03b5082a0e801df3eb107e5f3127a58693
Merge "Bug 51090: Render invalid wikilinks as plain text"