Remove jquery.scrollTo library

This is an old library that is broken in Chrome 61.
Rather than updating it we can just use $(window).scrollTop/Left
which works in all the browsers we currently support.

Fix MPSP.toggle to return a promise, instead of a deferred
(although this return value is not used yet).

Bug: T173618
Change-Id: Iaec56debbb808bd95cf65c1ebfe22bdf0116da68