Add DOI field

This is one of the subtasks for #56 (closed).

Add a unique DOI field (doi varchar(255) null) to the accession record. Accession historic should also have the field, but it should not have the unique constraint. Make sure database changes are registered in liquibase changelog.

Further:

1. Extend the AccessionJSON with the doi field so the field gets indexed.
2. Add display of DOI field to the accession details
3. Add display of DOI field to the accession explorer
4. Add DOI as filter (exact filter) to the accession filters