Send email when applicant is accepted, rejected, no availability,

[koenaguptaa05]

Motivation:
We need to send automated emails to the applicants when their application status is updated.

Acceptance Criteria:

• When the status changes to accepted, rejected, and no availability - Call the email controller endpoint to send an email to their email address. Use the text from the email templates below:
  https://docs.google.com/document/d/1VPHZQ9VEIvTQv7dafOaBF1Z9z_kyqh2RlxxxiCleJbU/edit?usp=sharing
• To test this - create or change an application entry in your database to use your own email. Then use postman to call the change status endpoint - and wait for an email to be received in your inbox.
• Also create unit tests for this - you should mock the email service for this purpose.