refactor: DJANGOCMS plugins settings (off stable)

[wesleyboar]

Overview

Move settings for plugins that are specific to Django CMS to _settings/.

Related

• mimicked by refactor: DJANGOCMS plugins settings (off main) #1019

Changes

• adds _settings/djangocms_plugins.py
• changes settings.py (removes code)

Testing

• Verify starting a CMS does not have errors.
• Verify Icon plugin has icons available and working.
  ^{Because BASE_DIR was replicated for DJANGOCMS_ICON_… settings.}

Warning

Mostly relying on verifying this is just moving code.

UI

Skipped.