nextcloud: prevent out of memory on big files upload/downloads

This should prevent issues with out of memory state when uploading or downloading big files. The Lighttpd stores request to temporally file (in default in /var/tmp) which on OpenWrt is to RAM and thus it can trigger out of memory case. This instead uses directory in nextcloud's own root. The path should be mounted on external storage using storage plugin but this change makes it even more essential that is is the case.

Reference to the original suggestion: #665 (comment 225881)

Fixes #665 (closed).