make html to plaintext conversion explicit

Several places in the codebase need a plaintext version of
post content for a snippet used in recent changes, history, etc.
Most were not using html_entity_decode though, which lead to
double encoding.  This consolidates those calls to all use the
same function in Parsoid Utils.

Change-Id: Ifcda2fa399b9920faa42a9ebf77ce31d0b2d0984