Add internship applicants count to internship listings.

**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.