nginx: Allow extending config with overrides

To facilitate this, the default redirect at / can be disabled, even if
the default remains at redirecting to the webmailer.

The extensions are within the host scope and are read from
$ROOT/overrides/nginx/*.conf.

core/nginx/conf/nginx.conf

@@ -84,14 +84,16 @@ http {
       }
       {% else %}
 
+      include /overrides/*.conf;
+
       # Actual logic
-      {% if WEBMAIL != 'none' %}
-      {% if WEB_WEBMAIL != '/' %}
+      {% if WEBROOT_REDIRECT %}
       location / {
-        return 301 {{ WEB_WEBMAIL }};
+        return 301 {{ WEBROOT_REDIRECT }};
       }
       {% endif %}
 
+      {% if WEBMAIL != 'none' %}
       location {{ WEB_WEBMAIL }} {
         {% if WEB_WEBMAIL != '/' %}
         rewrite ^({{ WEB_WEBMAIL }})$ $1/ permanent;