N°5621 Move showdown 2.0.0-alpha1 to NPM

Current file header is : /*! showdown v 2.0.0-alpha1 - 25-09-2018 */

Official 2.0.0-alpha doesn't match neither on content nor date :(
Considering date we would be at 1.8.7, but it is also quite different (and has vulnerabilities)
In consequence switching to 2.0.0.

package.json

@@ -20,6 +20,7 @@
     "mousetrap": "^1.6.5",
     "scrollmagic": "^2.0.8",
     "selectize-plugin-a11y": "^1.1.0",
+    "showdown": "^2.0.0",
     "tippy.js": "^6.2.5",
     "toastify-js": "^1.12.0"
   }