Remove 'Delete conversation' button

This PR removes the 'Delete conversation' option from the left-side menu on each conversation page. I also found the code for deleting a conversation, and I've added a note to have this functionality execute when a conversation no longer has any users attached to it. I think this code can be utilized when we start implementing the 'Leave conversation' functionality as an additional check.
One tiny nit: s/TOOD/TODO  ;)