Gets rid of manual XML generation.

Fixes #13002

Move the cloud/users/{userid} code in total to the provisioning API.

This reverts commit 6a1a4880.

This changeset removes the static class `OC_Request` and moves the functions either into `IRequest` which is accessible via `\OC::$server::->getRequest()` or into a separated `TrustedDomainHelper` class for some helper methods which should not be publicly exposed.

This changes only internal methods and nothing on the public API. Some public functions in `util.php` have been deprecated though in favour of the new non-static functions.

Unfortunately some part of this code uses things like `__DIR__` and thus is not completely unit-testable. Where tests where possible they ahve been added though.

Fixes https://github.com/owncloud/core/issues/13976 which was requested in https://github.com/owncloud/core/pull/13973#issuecomment-73492969

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.