Hygiene: Button style cleanup on diff view

Reverts changes to button.less introducing in in c5c6558 and provides an
alternative fix for Thank you button styling. Removing scoping from
current button styling would cause a lot of redundant CSS in new
overlays.

Change-Id: I723e17d14d8046a2dba40d8f76598d7b3eed5fac
Merge "Hygiene: Button style cleanup on diff view"