Backend translates the search results page

Now the search results page is presented by the backend in the language
controlled by the value of the `userlang` URL query parameter (or, if
the latter is missing, the value of the `Accept-Language:` HTTP header).

Note that the front-end doesn't yet take advantage of this
functionality.

src/search_renderer.cpp

@@ -192,7 +192,6 @@ kainjow::mustache::data buildPagination(
 
 std::string SearchRenderer::renderTemplate(const std::string& tmpl_str, const NameMapper& nameMapper, const Library* library)
 {
-  const std::string userlang("en");
   const std::string absPathPrefix = protocolPrefix;
   // Build the results list
   kainjow::mustache::data items{kainjow::mustache::data::type::list};