Rename 'fields' block in generic/form.html template to form_content

This was only recently introduced in
08fd30cb65 (not yet released).

We already have a documented 'fields' block in groups and users' create
and edit views for customisation. Using the same name for the overall
form content (other than the actions) would prevent us from making use
of this block for those documented templates.

Rename the block to form_content, so we can override the block while
also defining the documented 'fields' block in groups and users' create
and edit views.

wagtail/admin/templates/wagtailadmin/generic/form.html

@@ -32,7 +32,7 @@
     >
         {% csrf_token %}
 
-        {% block fields %}
+        {% block form_content %}
             {% if panel %}
                 {{ panel.render_form_content }}
             {% else %}

wagtail/documents/templates/wagtaildocs/documents/edit.html

@@ -1,7 +1,7 @@
 {% extends "wagtailadmin/generic/edit.html" %}
 {% load i18n wagtailadmin_tags %}
 
-{% block fields %}
+{% block form_content %}
     <div class="w-grid w-grid-cols-1 sm:w-grid-cols-6 w-gap-8">
         <div class="sm:w-col-span-5">
             <input type="hidden" value="{{ next }}" name="next">

wagtail/images/templates/wagtailimages/images/edit.html

@@ -10,7 +10,7 @@
     <script src="{% versioned_static 'wagtailimages/js/focal-point-chooser.js' %}"></script>
 {% endblock %}
 
-{% block fields %}
+{% block form_content %}
     <div class="w-grid w-grid-cols-1 sm:w-grid-cols-2 w-gap-8">
         <div>
             <input type="hidden" value="{{ next }}" name="next">