Only strip style attributes on rich paste from VE

Stripping all HTML atributes (to avoid CE-added styles such as
'font-size: 1em;') also strips data-parsoid which can cause
round trip errors. As an improvement only strip the style
attribute.

Bug: 58136
Change-Id: I34386bd847d1cf0583317a8b07916e43ff7af029
(cherry picked from commit 6a52fba64362d56a18617480b6098fc042059d45)
Merge "Only strip style attributes on rich paste from VE" into wmf/1.23wmf6