Invenio-db

- Pins Flask-SQLAlchemy below 2.5 due to breaking changes. Perhaps to revisit when fixed.

- Pins SQLAlchemy to >=1.2.18 and <1.4 due to incompatibility between
SQLAlchemy and SQLAlchemy-Utils.

- Hides password from output when running db init or db create.
- Disables MySQL 8 tests due to issue with Alembic

- Bump SQLAlchemy version to ``>=1.2.18`` to add support for PostgreSQL 12
- Integrate ``pytest-invenio`` and ``docker-services-cli`` for testing
- Support Python 3.8

- Deprecated Python versions lower than 3.6.0. Now supporting 3.6.0 and 3.7.0
- Use centrally managed Flask version (through Invenio-Base)
- Bumped SQLAlchemy version to ``>=1.1.0``
- SQLAlchemy-Utils set to ``<0.36`` due to breaking changes with MySQL
(``VARCHAR`` length)
- Enriched documentation on DB session management
- Stop using example app

- Unpin sqlalchemy-continuum
- Added tests for postgresql 10