Allow Postgres in addition to SQLite

[cmutel]

Allow for Postgres as a backend through setting environment variables:

• BW_DATA_POSTGRES: Should be truthy
• BW_DATA_POSTGRES_DATABASE: Postgres database name
• BW_DATA_POSTGRES_USER: Postgres username
• BW_DATA_POSTGRES_PASSWORD: Postgres password for username
• BW_DATA_POSTGRES_PORT: Port, default is 5432
• BW_DATA_POSTGRES_URL: Host URL, default is "localhost"

Current test implementation uses testcontainers, and tests on both Postgres and SQLite. The implementation isn't ideal as the container is rebuilt on each test, making them much slower than the SQLite tests. However, there are enough differences in the Peewee implementation that tests against Postgres are necessary, even in development.

Under Postgres there is no difference between projects, as we don't have separate database per project. This can be addressed later.

Only the tests in tests/database.py have been adapted - the switch from a decorator to a pytest fixture means either manual adaptation or some clever regex function needs to be applied.

• Remove SQLite-specific search functionality #1
• Remove bw2data parameterization #2
• Better approach to reuse Postgres test container via database reset or deleting data
• Design decision on whether test fixture approach is acceptable, and what strategy should be applied for porting tests
• Port other tests to new test approach