Add "COMPLETE" state for applications.

[mayura-andrew]

Description: A clear and concise description of the implementation.

Implement a new "COMPLETE" state for mentee applications, allowing admins to update the application status once the program has concluded.

Tasks: Add the tasks need to complete to achieve the implementation.

• Introduce a new "COMPLETE" status in the ApplicationStatus enum.
• Restrict the visibility and functionality of the "COMPLETE" button to admin users only.
• Implement functionality to send an email notification to the mentee when their application status is updated to "COMPLETE".
• Develop functionality to generate a completion certificate for the mentee.
• Ensure the certificate includes relevant details such as the mentee's name, program name, completion date, and any other pertinent information.
• Automatically attach the generated certificate to the email sent to the mentee.

Acceptance Criteria: Implementation should be able to perform these functionalities.

• Admins can change the application status to "COMPLETE" once the program is over.
• An email is sent to the mentee when their application status is updated to "COMPLETE".
• The email includes an attached certificate of completion for the mentee.

Additional Information: Add additional information

Related Dependencies or References: Add related dependencies and references relating to the implementation.

• Consult documentation or libraries for generating PDF certificates programmatically.