Fixing HTML validation errors when using Diazo

This pull request removes line feeds and carriage returns from meta description and adds upgrade step to do it for existing content. If this is not done the lxml transformation, done my diazo, can produce invalid HTML by escaping these characters. The bug cannot be fixed without plone.app.theming due to the problem happening in lxml.

See the http://bo.geekworld.dk/diazo-bug-on-html5-validation-errors/ and https://dev.plone.org/ticket/13871