fix: "Edit Page" links for multilingual sites

Fix #1232
Fix https://github.com/sourcethemes/academic-www/issues/14

layouts/partials/page_edit.html

@@ -1,7 +1,8 @@
 {{ if site.Params.edit_page }}{{/* Make `edit_page` optional in `params.toml` - prevent errors if it's removed. */}}
 {{ if site.Params.edit_page.repo_url | and (index site.Params.edit_page.editable .Type) | and (ne .Params.editable false) | or .Params.editable }}
+{{ $content_subfolder := cond site.IsMultiLingual (cond (ne .Site.LanguagePrefix "") (printf "%s/" .Site.LanguagePrefix) "") "" }}
 <p class="edit-page">
-  <a href="{{site.Params.edit_page.repo_url}}/edit/{{site.Params.edit_page.repo_branch | default "master"}}/content/{{.File.Path}}">
+  <a href="{{site.Params.edit_page.repo_url}}/edit/{{site.Params.edit_page.repo_branch | default "master"}}/content/{{$content_subfolder}}{{.File.Path}}">
     <i class="fas fa-pen pr-2"></i>{{ i18n "edit_page" | default "Edit this page" }}
   </a>
 </p>