Accession number should be unique

[pinarpink]

Describe the bug (current behaviour)

Several problems with the accession number field. It is defined as not nullable and with a default value of "-".
As a result when new partners/cohorts are added but not published they all get the accession number of "-".

This field should be nullable, without a default value. An entity should receive an accession number only when it is published.

To reproduce

1. Go to http://daisy.lcsb.uni.lu/api/partners
2. Partners with acronym "EURICE" and "FIBAO-GENYO" have the same accession number of "-"

Expected behavior

All partners delivered from the api endpoint should have a unique accession number.

Environment:

• Deployment: daisy.lcsb.uni.lu
• Version 1.2.0

Additional information (error logs, screenshots)

[vildead]

it will be solved by indetifier service