🔄 Refactor integrations

[mlamina]

This PR refactors the integrations page and organizes the dashboard views into separate files for better maintainability and readability.

Integrations Page Layout

• Improve the layout of the integrations page in dashboard/templates/integrations.html.
• Use a tabbed interface for Slack and Linear integrations.

Dashboard Views Refactoring

• Move views from dashboard/views.py to separate files: api_keys.py, integrations.py, stripe.py, and tasks.py.
• Update imports and URL patterns in dashboard/urls.py accordingly.

Task Events in Linear Integration

• Fix task events logging in linear_tools.py.
• Add task event logging for creating and searching Linear issues.

Unit Tests

• Add unit tests for Linear and Slack integration tools in engine/agents/tests/.
• Ensure tests use pytest and unittest.mock for mocking external dependencies.