Changes:

• stog-ocaml-session now handles options -w and -warn-error,
• fix Mathjax CDN URL in article template,
• provided article template and math module (math.stm) now use jquery.js and bootstrap-collapse.js (for foldable proofs),
• preview server:
  • use Ojs-base facilities to display messages in browser,
  • a "recompute" button on top-left corner allows the reload of all files (useful when adding new files to the edited web site sources),
  • use ppx_deriving_yojson 2.0 (and so require ocaml 4.02.1).