Refactor coding system database setup in AppConfig.ready()

[StevenMaude]

In Django 5 and up, we get the following warning:

…/django/db/backends/utils.py:98: RuntimeWarning: Accessing the database during app initialization is discouraged. To fix this warning, avoid executing queries in AppConfig.ready() or when your app modules are imported.
  warnings.warn(self.APPS_NOT_READY_WARNING_MSG, category=RuntimeWarning)

This is because of a call in AppConfig.ready().

That call in turn results in the following lines running that access the database:

• opencodelists/coding_systems/versioning/models.py

  Lines 62 to 68 in 772c560

  	def database_ready():
  	connection = connections[DEFAULT_DB_ALIAS]
  	connection.prepare_database()
  	executor = MigrationExecutor(connection)
  	targets = executor.loader.graph.leaf_nodes()
  	versioning_plan = executor.migration_plan(targets)
  	return not versioning_plan

• opencodelists/coding_systems/versioning/models.py

  Line 76 in 772c560

  for coding_system_release in CodingSystemRelease.objects.all():

Commenting these out breaks the code, but suppresses the warning:

def update_coding_system_database_connections():
    """Add the database config for each coding system release"""
    # ensure that the database is ready and the CodingSystemRelease table is available
    # (i.e. migrations have been run)
    # Remove database_ready() check from original code as that access the database:
    # if database_ready():  # pragma: no cover
    # The original code iterates through CodingSystemRelease.objects.all()
    for coding_system_release in []:
        if not CODING_SYSTEMS[coding_system_release.coding_system].has_database:
            continue
        db_path = (
            settings.CODING_SYSTEMS_DATABASE_DIR
            / coding_system_release.coding_system
            / f"{coding_system_release.database_alias}.sqlite3"
        )
        database_dict = {
            **connections.databases[DEFAULT_DB_ALIAS],
            **dj_database_url.parse(f"sqlite:///{db_path}"),
        }
        connections.databases[coding_system_release.database_alias] = database_dict

This is not something that currently breaks anything, but could in future if the warning behaviour is forbidden by Django in future. It's also discouraged by the relevant Django documentation, at least for Django 5.1.

[StevenMaude]

Options to deal with this from the Django 5.1 upgrade discussion:

• Rearchitect the coding system databases so that this initialisation is not needed.
• Rework this code to defer updating connections at a later time.
• Disable the behaviour in testing.