Merge branch 'dev-explorer-fixes' of ssh://gitlab.iscpif.fr:20022/gargantext/purescript-gargantext into dev-refact

* Added a "chevron" arrow to indicate open or closed status. This
  technically resolves the ticket, but I think it now makes it too
  busy and some further design work is necessary. (Hence "first
  draft".)
* Minor refactor to generate more correct UL/LI HTML hierarchy.
* Rip-and-replace nasty CSS hacks in index.html now that UL/LI is
  sane.

Still in TODO: refresh graph properly, show that it is updating