After clicking Expand All, Gerrit begins to render the diffs and if
the user starts scrolling, the 'never' scroll behaviour is supposed to
prevent the scrolling of the user to the top.
We prevent any auto scrolling happening now after Expand All is
clicked.
This however introduces a regression in which the first chunk is not
focused on a small screen but we are fine in not supporting this use
case as of now.
Rename ScrollBehaviour to ScrollModes as Behaviours in Gerrit are
similar to mixins.
Move ScrollModes into constants.js
Change-Id: Ic904c01c5bf93934c8c9b215e35e4b1d95f4498d