feature: Short abstracts and show less/more button

* Shortified versions of abstracts will get shown by default.

* Added a Show more/less button if necessary

* When clicking on the button its state will toggle and so will the
  abstract (from short to original and vice versa)
update: A better limit for short abstracts

* Added DDG_SHORTABSTRACT_LEN to facilitate the process of keeping track
  and changing the limit in the future.