To prevent unexpected behavior, public.php and the OCS API calls will
return 503 Service Unavailable when an upgrade is due.

We should use the proper content-type `application/xml` instead of the default `text/html` here.

Backport requested.

 /ocs/cloud/user
Response:
 <?xml version="1.0"?>
 <ocs>
  <meta>
   <status>ok</status>
   <statuscode>100</statuscode>
   <message/>
  </meta>
  <data>
   <id>thomas</id>
   <display-name>DeepDiver</display-name>
   <email>no-response@domain.tld</email>
  </data>
 </ocs>

remove the activity call here. it is not implemented anyways. This will be provided by Activity app in the future.

revert changes to fbbc76f2 because master is very unstable right now

Add support for a reverse proxy that handles multiple domains via different
web roots (http[s]://proxy.tld/domain.tld/owncloud).

As the reverse proxy web root is transparent for the web server the
REQUEST_URI and SCRIPT_NAME need manual adjustments. This patch replace
the direct use of this _SERVER variables with function calls and extend
this functions to overwrite the web root. Additionally it adds a Sabre
request backend that extends the Sabre_HTTP_Request to use the same
functions.

make it possible to manually override the hostname and protocol if the automatic detection from ownCloud fails. This can happen in reverse proxy situations or with loadbalancers setups.