Add demo mode functionality with mock data support
- Introduced a new boolean setting 'demo_mode' in the schema to enable mock data usage.
- Created a new service 'PiwikApiDemoClient' to handle mock data generation for various API endpoints.
- Updated 'PiwikApiClient' to utilize the demo client when demo mode is enabled.
- Added a new enum 'MockModes' to define different mock data generation modes.
- Modified the 'ApiController' to handle exceptions more cleanly. (php linter errors)
- Enhanced the 'SettingsForm' to include a checkbox for enabling demo mode.
- Refactored existing methods to integrate mock data responses based on the selected mode.
Closes #3575164