[feature] As a mentee, I would like to be able to decline an invitation

[polina-munoz]

Currently a mentor can delete an invitation, but a mentee cannot request to decline an invitation without contacting the mentor first via email/chat.

Suggestion for User flow:

1. Mentor creates invitation.
2. Mentee gets a confirmation email.
3. Mentee has ability to decline invitation in confirmation email.
4. An email is sent to the mentor signifying this change.
5. Invitation is deleted.