Use new footer_api_host setting

Ref readthedocs/readthedocs.org#6131

Author: stsewd

readthedocs_ext/_static/readthedocs-data.js_t

@@ -10,6 +10,7 @@ var READTHEDOCS_DATA = {
     docroot: {{ conf_py_path|default('')|tojson }},
     source_suffix: {{ source_suffix|default('')|tojson }},
     api_host: {{ api_host|default('')|tojson }},
+    proxied_api_host: {{ proxied_api_host|default('')|tojson }},
     commit: {{ commit|default('')|tojson }},
     ad_free: {% if ad_free %}true{% else %}false{% endif %},