fix https://github.com/sagemathinc/smc/issues/843: "SMC public html page shows briefly in firefox window, then vanishes"
fix https://github.com/sagemathinc/smc/issues/843: "SMC public html page shows briefly in firefox window, then vanishes"

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