DAR-2107 | Abstract the fluid background logic

@Wikia/darwin-team 

Branch: DAR-2107

Todo changes in theme designer:
1) Instead of passing *background-fixed*, *background-tiled* as params we now just need to pass *background-classes=true*
2) There are new classes that need to be added to body element to make them work:
a) *background-fixed* - it's the same as background-fixed setting in Theme Designer
b) *background-not-tiled* - it's NEGATION of option background: tiled from Theme Designer
c) look below

Also: DAR-2395 Determine how to create a background that works in fluid but never needs to split
This is done by adding (or not adding) *background-dynamic* class to body element

Also: DAR-2400: Allow two background colors to be defined for fluid backgrounds - middle split and edges
Added option to pass *color-middle* to sass params and it will fill center of dynamic background

Also: DAR-2627: Create css class for AdSkin background - adding javascript logic for AdSkin and change AdTemplateSkin for AdEng.

**DO NOT MERGE**