Add ability to deactivate account

[mblayman]

Users need the ability to deactivate their account, whether at the end of their trial, or when they no longer plan to use College Conductor. I don't want to have to do account deactivation manually.

• Add deactivate section to settings page.
• Add form to handle deactivation.
• Create deactivated page.
• Log user out.
• Handle inactive state so they can't continue to use the service.
• Cancel subscription at Stripe