Fix #1522: fix 'str' object has no attribute 'attributes' (#1528)

* Fix #1522: fix attribute error if css is str

* Revert "Fix #1522: fix attribute error if css is str"

This reverts commit b22a77f.

* Use `css_tag` only for standard CSS files from Sphinx

... and keep using the custom `link` HTML tag for CSS files included via the
HTML theme option `extra_css_files`.

* Normalize template variables

---------

Co-authored-by: Manuel Kaufmann <humitos@gmail.com>
Co-authored-by: Anthony Johnson <aj@ohess.org>

sphinx_rtd_theme/layout.html

@@ -28,12 +28,13 @@
     <link rel="stylesheet" href="{{ pathto('_static/' + style, 1) }}" type="text/css" />
     <link rel="stylesheet" href="{{ pathto('_static/pygments.css', 1) }}" type="text/css" />
   {%- endif %}
-  {%- for css in css_files %}
-    {{ css_tag(css) }}
+  {%- for css_file in css_files %}
+    {{ css_tag(css_file) }}
   {%- endfor %}
 
-  {%- for cssfile in extra_css_files %}
-    {{ css_tag(cssfile) }}
+  {# "extra_css_files" is a theme option and it's always a string #}
+  {%- for css_file in extra_css_files %}
+    <link rel="stylesheet" href="{{ pathto(css_file, 1)|escape }}" type="text/css" />
   {%- endfor -%}
 
   {#- FAVICON