Add post-install script to copy bootstrap files in theme dir

As @PhrozenByte pointed out, we block access to the `vendor` folder
in our `.htaccess` so accessing the assets directly is very likely to
cause problems.

composer.json

@@ -24,5 +24,11 @@
             "PicoPluginInterface": "lib/",
             "AbstractPicoPlugin": "lib/"
         }
+    },
+    "scripts": {
+        "post-install-cmd": [
+            "cp vendor/twbs/bootstrap/dist/css/bootstrap.min.css themes/default/bootstrap.min.css",
+            "cp vendor/twbs/bootstrap/dist/js/bootstrap.js themes/default/bootstrap.js"
+        ]
     }
 }