BibEdit: use tmp-shared space for temporary files

• In a load-balanced multi-node setup, some temporary files need to be shared between worker nodes. This commit introduces new tmp-shared space that exists alongside the usual worker-node-specific tmp space. The tmp-shared space is to be mounted on some AFS or NFS shared filesystem.

• Adapts BibEdit and BibEditMulti to use the tmp-shared space. (closes #430)

• Adds tmp-shared space (CFG_TMPSHAREDDIR) alongside node-specific tmp space (CFG_TMPDIR) to the list of FFT-allowed directories for bibupload/bibdocfile.