Connector export: add backend infrastructure code
Connector export: specify output directory [settings update]

A new settings variable MEDIA_CONNECTOR_SUBDIRECTORY is introduced to specify a
subdirectory of MEDIA_ROOT in which created connector archives are stored. The
back-end checks if this folder is accessible and returns an error if not.
Move random ID generator to common module
Connector export: add connector retrieval and output infrastructure

This commit adds the retrieval of many or single connectors (if a sample is
wanted), the creation of an output folder with a random name and based on this a
tar.gz file. Nothing is added to the output folder/tar file, yet.
Connector export: create connector folder structure

This commit adds code so that the folder structure for every exported
connector is created. This folder structure is:

  output_path/neuron_id/[presynaptic|postsynaptic]/connector_id

In case a connector is not linked with the presynaptic_to or the
postsynaptic_to relation of the current project, a folder named
'unknown_<relation_id>' is created instead.