Get TinyMCE back to work

The following discussion from !29 (merged) should be addressed:

• @grote started a discussion:

  Let's drop the revert commit from this MR.

  The easiest (temporary) alternative would be to just re-add the mdl class it removed.

  Better would be to find a work-around for pages that include TinyMCE. You can try those:

  • https://github.com/google/material-design-lite/issues/1369#issuecomment-242333651
  • https://github.com/tinymce/tinymce/issues/2395#issuecomment-168016133
  • https://github.com/S--Minecraft/s--minecraft.github.io/commit/fdbafd74dc49c5dff130fefbc7dbe73bbe6d9f13
  • http://stackoverflow.com/a/40663208