Background color should be #F9F9FA (Grey 10) including migration

[aaronrbenson]

[Mardak]

@aaronrbenson what should the background be for various other items:

manual migration (here grey-20):

link menu (here grey-20):

confirmation/edit topsites modal (here white and gray-out background grey-20 80%) :

prefs pane (here white) and subitem (here grey-20):

hovered top sites buttons (here grey-20):

info option description (here white):

pocket article (here white):

[Mardak]

Updated Spec: https://mozilla.invisionapp.com/share/MHD92KAYR

And aaron says grey-10 for link menu and also grey-10 for pref options. Other backgrounds above are good