Add in Mixpanel activity tracking

[ben-coo-per]

• [x] Add mixpanel util
• [x] Add tracking to Auth pages
• [x] Add tracking to event creation & action pages
• [x] Add tracking to account pages
• [x] Add ability to opt-out in settings

[ben-coo-per]

I'm moving "Add popup alert about tracking ~" into a separate issue that is dedicated to a messaging system.

[ben-coo-per]

It looks like the mixpanel events are being spread out across different entities for each page... I need to do some aliasing

• [x] Add aliasing to keep user events associated with the same user