layouts: use details for the post informations

* Wraps the post informations inside a details HTML tag.
* Reduces the bottom margin of the post main heading.
* Reorders the SCSS rules in order to set the post-info class to the
  details HTML tags.
* Add the permalink to the post informations.
* Uses the i18n mechanism for the new layout strings, such as
  "post-infos" or "permalink".
* Updates the changelog.
* Closes #17.

Co-Authored-by: iGor milhit <igor@milhit.ch>