webapp/lib: polyfill for String.prototype.includes -- issue #1874
webapp/lib: polyfill for String.prototype.includes -- issue #1874

DrXyzzy/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc,DrXyzzy/smc,sagemathinc/smc,tscholl2/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc,sagemathinc/smc,tscholl2/smc