Icon updates and testings

[tdinh33]

• Update the client’s goal model to include icons and display them on goal cards.
• Implement an emoji picker and modify the Goal manager for users to add/change icons and submit the code file containing these changes.
• Complete the implementation of the pickEmojiOnClick event handler in the React frontend.
• Write unit tests to cover the GetGoalsForUser route in the GoalController using C#