Cherry pick of Bug 1369830 for the upgrade of html purifier to 4.10.0 


As we add a new configuration option to HTML Purifier,
we have to update HTMLPurifier/ConfigSchema/schema.ser
 1. Download and unzip HTMLPurifier package
 2. Copy Filter.ExtractStyleBlocks.PreserveCSS.txt
from mahara to the unzip folder
 3. Run php maintenance/generate-schema-cache.php in
unzup folder
 4. Copy over lib/HTMLPurifier/ConfigSchema/schema.ser
from unzip folder to mahara

behatnotneeded

Change-Id: I42618f0998425d20f7732843af8a700260c8d96e