improvements to export summary
improvements to export summary

* fix #1495 - remove duplicate ,
* use HTML5 charset attribute
* add the current Snap! language to the html tag

jmoenig/Snap--Build-Your-Own-Blocks,ericrosenbaum/BeetleBlocks,ericrosenbaum/BeetleBlocks,jmoenig/Snap--Build-Your-Own-Blocks