Modernise the edit-this-page.html template

This now uses the current `basic-ng` template for determining when
and how to present the edit button.

src/furo/theme/furo/components/edit-this-page.html

@@ -1,3 +1,4 @@
+{% extends "basic-ng/components/edit-this-page.html" %}
 {% from "basic-ng/components/edit-this-page.html" import determine_page_edit_link with context %}
 
 {%- macro furo_edit_button(url) -%}
@@ -13,15 +14,14 @@
 </div>
 {%- endmacro -%}
 
+{% block link_available -%}
+{{ furo_edit_button(determine_page_edit_link()) }}
+{%- endblock %}
 
-{%- if theme_source_repository -%}
-  {%- if not theme_source_branch -%}
-    {{ warning("Provided `source_repository` but not `source_branch`. ")}}
-  {%- endif -%}
-  {%- if page_source_suffix -%}
-    {{ furo_edit_button(determine_page_edit_link()) }}
-  {%- endif -%}
-{%- elif READTHEDOCS and conf_py_path and page_source_suffix and github_user != "None" and github_repo != "None" and github_version %}
+{% block link_not_available %}
+{# Make nice things happen, on Read the Docs #}
+{%- if READTHEDOCS and conf_py_path and page_source_suffix and github_user != "None" and github_repo != "None" and github_version and pagename and page_source_suffix %}
   {% set url = "https://github.com/" + github_user + "/" + github_repo + "/edit/" + github_version + conf_py_path + pagename + page_source_suffix %}
   {{ furo_edit_button(url) }}
 {%- endif -%}
+{% endblock %}