Refer to relative path instead of absolute path

There is no need to refer to the absolute path here if we can use the relative one.

Conflicts:
	lib/private/templatelayout.php
Add unit tests for convertToRelativePath
Add "$_SERVER['REQUEST_URI']" to fix the unit tests

Let's hope that works
Add proper setup and teardown

Properly restore REQUEST_URI and SCRIPT_NAME after test runs
Merge pull request #11593 from owncloud/useRelativePathToOwnCloudInstallation

Refer to relative path instead of absolute path