Add ARCHES_APPLICATIONS to core settings.py

[jacobtylerwalls]

The ARCHES_APPLICATIONS setting is provided with the new project template and included in the project upgrade instructions, but as of #10613 (7.6 dev), we need to include it with core settings.py also so that setup_db runs. (To run the test suite, one must setup_db first.)

2024-03-26 10:10:41,045 arches.app.utils.data_management.resource_graphs.importer ERROR    'Settings' object has no attribute 'ARCHES_APPLICATIONS'
Traceback (most recent call last):
  File "/Users/jwalls/prj/arches/arches/app/datatypes/datatypes.py", line 101, in get_instance
    datatype_instance = DataTypeFactory._datatype_instances[d_datatype.classname]
                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
KeyError: 'BaseDataType'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/jwalls/prj/arches/arches/app/models/system_settings.py", line 67, in __getattr__
    return super(SystemSettings, self).__getattr__(name)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/site-packages/django/conf/__init__.py", line 104, in __getattr__
    val = getattr(_wrapped, name)
          ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Settings' object has no attribute 'ARCHES_APPLICATIONS'