Send URI instead of filepath to NGINX for X-Accel

Sends URI "**/WEBROOT?**/data/**USER**/files/**filename**" to NGINX instead of the full file path.

The NGINX section of the Sendfile documentation has already been updated for this commit as seen [here.](https://github.com/owncloud/documentation/blob/master/admin_manual/configuration/xsendfile.rst#configuration-2)

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Replaces https://github.com/owncloud/core/pull/7838
Fixes #7754