Issue #2235901 by alexpott, mdrummond, iMiksu, sun, Wim Leers: Remove custom theme settings from *.info.yml

Issue #2271529 by attiks, alexpott, Lowell, YesCT, Jelle_S | mdrummond: Move breakpoint settings to theme and module *.breakpoints.yml files at root level.

Issue #2233787 by alexpott, xjm | tim.plunkett: Move default configuration into extension/config/install to clarify its purpose.

Issue #2218655 by sun, effulgentsia, jessebeach: Core expects $theme.settings default config file to exist.

Issue #2167623 by danilenko_dn, sidharthap, Nitesh Sethia, krishnan.n, aitiba, alexpott, ashwinikumar, Barrett, damiankloip, deepakaryan1988, foxtrotcharlie, ianthomas_uk, neetu morwani, nonsie, piyuesh23, Sharique, sivaji, sushantpaste, swentel, vijaycs85, YesCT: Add test for all default configuration to ensure schema exists and is correct.

Issue #1712250 by alexpott, n3or, cam8001, vijaycs85, Albert Volkman, jibran, heyrocker, chx: Convert theme settings to configuration system.