Expired Links and Invitation Functionality

[yatharth-b]

Resolves #174

Added a pub/sub function to check for expired links every week which deletes invites from the invites collection if they are expired and also deletes the invites from the sender's schedule collection.

Checkllist

• A new field is added to the friend-invites collection/schema to track time.
• If an expired invite link is clicked, it should return some response and delete the friend from the invited list in the schedules collection.
• A new cloud function is added to clean up expired invitations every 7 days.

How to test

Start the cloud function emulator and send a request from the front end using the invitation modal.
I was not able to test the pub/sub function but it should work as expected because it is very similar to the code in the handle invitation function.