Invenio-records

- Fixes a bug on MultiRelationsField where fields would be calculated multiple
times. Now they are calculated only once, and cached.

- Adds support for nested RelationFields via MultiRelationsField
- Migrates to declarative module (setup.cfg)
- Migrates code to Black formatter
- Adds german translations
- Removes babel extensions for Jinja

- Adds utility to merge Python dicts

- Fixes regression in 1.7.1.

- Fixes dictionary lookup during dereferencing for optional/non-existing keys.

- Adds support for relations defined via database-level foreign keys.

- Backwards incompatible: Changes the relations definitions to support both
dictionary keys and object attributes instead of only dictionary keys. Change
existing code from::

Relation(
...
attrs=['title'],
),

to::

Relation(
...
keys=['title'],
),

- Move from setup.py to setup.cfg for purely declarative package definitions.