Refactor to datalayer

Description

Closes issues: Related issues:

Refactor common modules dealing with loading/writing data to own datalayer package.

• Create datalayer/
• Move api/allelefilter/ to datalayer/
• Move creators/loaders from api/util/ to datalayer/
• Move relevant tests (still run in API tests so we test on migrated database)
• Remove unused sanger_verification module
• Consolidate all different conftest.py into one root level
• Move to using --chown ella-user:ella-user to Dockerfile to decrease image build time (was manual chown for backwards compatibility, but people should have moved to newer Docker by now)

Notes to reviewer

Type of change

Application (affects UI or general functionality):

• New feature
• Bug fix
• Improvement

Ops / admin / CI related only (not impacting users):

• New feature
• Bug fix
• Improvement

Tests

General

• Tests have been added that prove my fix is effective or that my feature works
• Related tests have been modified/removed

Hypothesis testing:

• Soak testing has been done
• Distribution between positive / negative cases has been checked

Database

• Includes changes to database schema
• Includes necessary database migrations

Configuration

• Includes changes to configuration
• Includes configuration migration instructions in documentation

Merge checklist

• Self-review of code performed
• Feature review against specification (if applicable)
• Need for documentation has been evaluated and, if necessary, updated
• Code and implementation is reviewed by other core developer (all changes, inc. changes based on initial review)