Comment out mixpanel code; #686

I accidentally added mixpanel code on the username branch (see #287 and
 #680) by creating that branch while on the mixpanel branch. Now the
username/id stuff is deployed and our app is looking for MIXPANEL_TOKEN,
but I'm not ready to send stuff to Mixpanel yet. This simply comments
out the mixpanel calls I had added so far.
Bump version to 7.4.1
Bump version to 7.4.1-dev