Enable docsearch in development env.

Previously a search redirected to `https://getbootstrap.com/...`

_includes/docs-sidebar.html

 <form class="bd-search d-flex align-items-center">
-  <input type="search" class="form-control" id="search-input" placeholder="Search..." aria-label="Search for..." autocomplete="off">
+  <input type="search" class="form-control" id="search-input" placeholder="Search..." aria-label="Search for..." autocomplete="off" data-baseurl="{{ site.baseurl }}/docs/{{ site.docs_version }}">
   <button class="btn btn-link bd-search-docs-toggle d-md-none p-0 ml-3" type="button" data-toggle="collapse" data-target="#bd-docs-nav" aria-controls="bd-docs-nav" aria-expanded="false" aria-label="Toggle docs navigation">
     {%- include icons/menu.svg width="30" height="30" -%}
   </button>

assets/js/src/application.js

@@ -115,7 +115,8 @@
         },
         transformData: function (hits) {
           return hits.map(function (hit) {
-            hit.url = hit.url.replace('https://v4-alpha.getbootstrap.com', '/docs/4.0')
+            var baseurl = document.getElementById('search-input').getAttribute('data-baseurl')
+            hit.url = hit.url.replace('https://getbootstrap.com' + baseurl, baseurl)
             return hit
           })
         },