Added preference to control plugin usage

api/funkwhale_api/plugins/dynamic_preferences_registry.py

+from dynamic_preferences import types
+from dynamic_preferences.registries import global_preferences_registry
+
+plugins = types.Section("plugins")
+
+
+@global_preferences_registry.register
+class PluginsEnabled(types.BooleanPreference):
+    section = plugins
+    show_in_api = True
+    name = "enabled"
+    default = True
+    verbose_name = "Enable Funkwhale plugins"
+    help_text = "If disabled, all installed and enabled plugins will be ignored."