Use placeholders in GCWU default SCSS

Added TODOs for specificity
Use placeholders in GCWU default-screen SCSS

Added TODO for follow up
Follow up for IE sheet
Use placeholders in GCWU default-desktop-screen

TODO for later follow up
Use placeholders in GCWU default-desktop-screen-images SCSS

TODO for later followup
Use placeholders for GCWU default-mobile SCSS

Added TODOs for followup
Use placeholders for GCWU default-mobile-images
Update placeholders in GCWU default-mobile-ns

Fix namespacing of placeholder
Use placeholders for GCWU default-responsive-1200
Use placeholders for GCWU default-responsive-768
Use placeholders for GCWU default-print

Hoist native elements
Use placeholders for GCWU default-screen-images-ie

Merged IE 7 & 8 sheets since they were dups
Rebrace GCWU default-screen-images SCSS
Use placeholders for GCWU default-screen-ie7
Fixup b22600d
Incorrectly changed path for Intranet theme.
Bad semicolon in placeholder name
Use placeholders for GCWU default-screen SCSS
Use placeholders for GCWU serv SCSS
TODO for followup
User placeholders for GCWU serv-mobile

TODO for followup
Use placeholders for GCWU sp-pe-desktop-screen

Remove answered TODO
Use placeholders for GCWU sp-pe SCSS

Rename some old placeholders too