Added ownCloud backend for external storage

To make it possible to use the short ownCloud URL (without specifying
webdav.php or remote.php), a new backend is available for ownCloud.

The user must specify the host + context path in the "Url" field (which
is mapped to the "host" parameter) and the subdir to mount in the "Root"
field.

This is to prevent confusion because some users forget to append
webdav.php or remote.php to the WebDAV URL.

Fixes #4923
Moved content disposition code+workarounds to OCP\Response

Added new OC\Response API called setContentDispositionHeader() that
contains the needed workarounds for UTF8 and IE.

Refactored download code to use the new API.

Removed unused trashbin download file.
Added workaround for Android content disposition

Fixes #5807
Enforce required phpunit version

This will prevent frustration and confusion when unit tests fail because
the wrong phpunit version was used
Added isUserAgent() method to request

- added isUserAgent() method to OC_Request which makes it possible to
  test it
- OC_Response::setContentDisposition now uses OC_Request::isUserAgent()
Adding a random postfix to the part file.
enable ldap tests
avoid conflict with physical file config.js
new config parameter 'front_controller_active' which will instruct the url generator to generate urls without index.php
use getAppWebPath() in here as well
require composer's autoload.php if present
reference 3rdparty branch with SabreDAV 1.7.9 installed using composer
[tx-robot] updated from transifex
in order to work properly with encryption ocTransferId is added to the file path - questionable usage of magic string
Merge pull request #6530 from owncloud/prevent-corruption-on-parallel-upload-master

Adding a random postfix to the part file.
reuse existing helper function OC_Helper::is_function_enabled
we shall explode on ',' only
remove duplicate exe mimetype, add correct msi mimetype
Remove @ in order to get proper error handling
Merge pull request #6663 from owncloud/fix_empty_mimetypes

remove duplicate exe mimetype, add correct msi mimetype