[WIP] SASS compilation variable conventions update

SASS naming convention updates for baseline variable usage on padding and margins, ui-depth extends, along with other small fixes. 

The intent with this is to not have any user-facing impact, and instead move us closer toward having files that use our newer sass conventions.

Currently Includes:
1. - #fff; --> $white;
2. - #000 ---> $black;
3. - margin/padding properties --> multiples of $baseline when possible
4. - replacing z-index with %ui-depth extends
5. - moving multipliers less than 1 (ex: .5, .75, etc) to have 0 prefix (ex: 0.5, 0.75, etc) 
6. - #f2f2f2 & #f3f3f3 --> $gray-l5;
7. - #f8f8f8; --> $gray-l6;
8. - define second amounts are now all using the timing variables defined for both the LMS and CMS. 
9. - rgba shadows are now all using the shadow variables if they line up to existing options. 
10. - %ui-fake-link extend applied where appropriate
11. - %cont-no-list extend applied where appropriate

Next:
- Will migrate other existing white, black, and gray hex shades over to use our current color variables where possible. 
- FED review. 
- Other suggestions for quick fixes? (Feel free to add on any other commits here as well if that's easiest.) 

Future Cleanup:
- Property ordering removed from this in order to limit potential cases where property ordering cleanup would change styles. 


Primary Reviewer: TBD 
Interested Parties: @talbs, @frrrances, @clrux, @andywaldrop. 
@clrux, I definitely agree, but I think gutter widths would be best tied to a definitive grid system (with vertical rhythm in this case). Until we move Studio or our apps to a (responsive) grid system, I'm fine with vertical spacing being dictated by ``$baseline`` or if we want to create a separate ``$gutter-vertical`` variable.