upgraded next / SASS support

[mgiraldo]

this is going to be one of those massive branches because it will affect every html file in the repo so i decided to make this first push

• upgraded to next 5.1
• added support to sass while still using the old postcss which i am deprecating (this was the hardest thing to figure out... how to have both coexist for now)
• migrated the header and footer from postcss to sass

main advantages:

• being in the latest version of next
• will eventually get rid of the skeleton-loader module
• simplify the deployment process when we remove many postcss-related modules once the migration is complete
• less use of dangerouslySetInnerHTML
• source mapping working (not there yet but should be eventually)
• better syntax checking in IDE

[mdellabitta]

Oooh, source mapping working

[mgiraldo]

ok i'm done with the last set of migrations:

• no more warnings related to versions of packages having “unmet peer dependencies”
• latest carousel version (remember when it was right-aligned? upgrading everything seems to have fixed that, see also above)
• no more postcss \o/
• source maps working! 🎉
• all the benefits of next 5.1

[mgiraldo]

could @mdellabitta @AudreyAltman @markbreedlove someone give this an additional sanity check before i merge? basically clicking around making sure things work fine.

[mdellabitta]

I clicked around on my local and things looked good.