Convert /update_lang/ page to use Bootstrap

LEARNER-1922

cms/templates/darklang/preview_lang.html

@@ -1,15 +0,0 @@
-## Override the default styles_version to the Pattern Library version (version 2)
-<%! main_css = "style-main-v2" %>
-<%page expression_filter="h"/>
-<%!
-from django.utils.translation import ugettext as _
-%>
-
-<%inherit file="../base.html" />
-<%block name="title">${_("Preview Language Setting")}</%block>
-<%block name="bodyclass">is-signedin pattern-library</%block>
-
-<%block name="content">
-    <%include file="/dark_lang/preview_lang_include.html" />
-</%block>
-