Add internship applicants count to internship listings.

[JahagirdarPrajwal]

Description:
Display the number of applicants for each internship.

Tasks:

1. Track the count of applicants for each internship.
2. Update the count when a new application is submitted.
3. Show the total number of applicants on the internship listing.

Expected Outcome:
Users and admins can see how many people have applied for each internship.