Use Sanitizer::stripAllTags( $x ) instead of html_entity_decode( strip_tags( $x ) )

Use Sanitizer::stripAllTags( $x ) instead of html_entity_decode( strip_tags( $x ) )

We have a utility function for this, so let's use it.

What I don't understand though is why Sanitizer uses custom PHP implementations
for both tag stripping and entity decoding, instead of the built-in functions.
If there's a security reason for this or the built-ins are inadequate, that's
fine, but then that should be documented (and we should possibly ban usage
of the built-ins).

Change-Id: I2ba2ecd388cb3d9cd2360ecaa236f3d444f0eabf