Invalidate Ticket - Backend Support

[victorzheng02]

When a user issues a delete on the frontend (see #99), we wish to invalidate (soft delete) the corresponding ticket and its children. This way we can still recover the data. Use a plugin such as https://github.com/dsanel/mongoose-delete for this.

Permissions: If the user is the owner of the ticket or they have higher permissions than director, they are allowed to invalidate the ticket.