Closes #9623: Implement saved filters (#10801)

* Initial work on saved filters

* Return only enabled/shared filters

* Add tests

* Clean up filtering of usable SavedFilters

netbox/extras/forms/mixins.py

@@ -0,0 +1,73 @@
+from django.contrib.contenttypes.models import ContentType
+from django import forms
+
+from extras.models import *
+from extras.choices import CustomFieldVisibilityChoices
+from utilities.forms.fields import DynamicModelMultipleChoiceField
+
+__all__ = (
+    'CustomFieldsMixin',
+    'SavedFiltersMixin',
+)
+
+
+class CustomFieldsMixin:
+    """
+    Extend a Form to include custom field support.
+
+    Attributes:
+        model: The model class
+    """
+    model = None
+
+    def __init__(self, *args, **kwargs):
+        self.custom_fields = {}
+        self.custom_field_groups = {}
+
+        super().__init__(*args, **kwargs)
+
+        self._append_customfield_fields()
+
+    def _get_content_type(self):
+        """
+        Return the ContentType of the form's model.
+        """
+        if not getattr(self, 'model', None):
+            raise NotImplementedError(f"{self.__class__.__name__} must specify a model class.")
+        return ContentType.objects.get_for_model(self.model)
+
+    def _get_custom_fields(self, content_type):
+        return CustomField.objects.filter(content_types=content_type).exclude(
+            ui_visibility=CustomFieldVisibilityChoices.VISIBILITY_HIDDEN
+        )
+
+    def _get_form_field(self, customfield):
+        return customfield.to_form_field()
+
+    def _append_customfield_fields(self):
+        """
+        Append form fields for all CustomFields assigned to this object type.
+        """
+        for customfield in self._get_custom_fields(self._get_content_type()):
+            if customfield.ui_visibility == CustomFieldVisibilityChoices.VISIBILITY_HIDDEN:
+                continue
+
+            field_name = f'cf_{customfield.name}'
+            self.fields[field_name] = self._get_form_field(customfield)
+
+            # Annotate the field in the list of CustomField form fields
+            self.custom_fields[field_name] = customfield
+            if customfield.group_name not in self.custom_field_groups:
+                self.custom_field_groups[customfield.group_name] = []
+            self.custom_field_groups[customfield.group_name].append(field_name)
+
+
+class SavedFiltersMixin(forms.Form):
+    filter = DynamicModelMultipleChoiceField(
+        queryset=SavedFilter.objects.all(),
+        required=False,
+        label='Saved Filter',
+        query_params={
+            'usable': True,
+        }
+    )