Dynamically set form action URL

Homologous to
https://framagit.org/staticman-gitlab-pages/huginn/commit/7f91aaf7add8a8c2528b513a9ef9315cd2ae9db3.
See https://www.willmaster.com/library/manage-forms/bot-block-for-forms.php for
rationale.

_includes/staticman-comments.html

@@ -20,7 +20,7 @@ <h3 class="page__comments-title">{{ site.data.ui-text[site.locale].comments_titl
     <div class="page__comments-form">
       <h3 class="page__comments-title">{{ site.data.ui-text[site.locale].comments_label | default: "Leave a Comment" }}</h3>
       <p class="small">{{ site.data.ui-text[site.locale].comment_form_info | default: "Your email address will not be published. Required fields are marked" }} <span class="required">*</span></p>
-      <form id="new_comment" class="page__comments-form js-form form" method="post" action="{{ site.staticman.endpoint | default: 'https://staticman3.herokuapp.com/v3/entry/github/' }}{{ site.staticman.repository }}/{{ site.staticman.branch }}/comments">
+      <form id="new_comment" class="page__comments-form js-form form" method="post">
         <div class="form-group">
           <label for="comment-form-message">{{ site.data.ui-text[site.locale].comment_form_comment_label | default: "Comment" }} <small class="required">*</small></label><br>
           <textarea type="text" rows="12" cols="36" id="comment-form-message" name="fields[message]" tabindex="1"></textarea>

js/staticman.js

@@ -1,3 +1,7 @@
+---
+layout: null
+---
+
 (function ($) {
   var $comments = $('.js-comments');
 
@@ -6,9 +10,14 @@
 
     $(form).addClass('disabled');
 
+    {% assign sm = site.staticman -%}
+    var endpoint = '{{ sm.endpoint | default: "https://staticman3.herokuapp.com/v3/entry/github/" }}';
+    var repository = '{{ sm.repository }}';
+    var branch = '{{ sm.branch }}';
+
     $.ajax({
       type: $(this).attr('method'),
-      url: $(this).attr('action'),
+      url: endpoint + repository + '/' + branch + '/comments',
       data: $(this).serialize(),
       contentType: 'application/x-www-form-urlencoded',
       success: function (data) {