Fix #4785: Add purge queue API and frontend UI integration with tests

[Harsh-6291]

Description

This pull request includes the following changes:

• Added a new API method to purge the submission queue for challenges, accessible only by challenge hosts.
• Integrated frontend UI components including:
  • A "Purge Queue" button visible to hosts on the submissions page.
  • A confirmation modal dialog allowing hosts to confirm queue purge actions.
• Modified Angular services to include the API call for purging the queue.
• Added comprehensive backend tests for the purge queue API endpoint.
• Updated apps/challenges/urls.py and views.py to support the new purge queue functionality.
• Enhanced frontend submission components (challengesubmissions.component.html) with the purge queue UI.
• Complied with coding standards and ensured consistent style.