Implement multitenancy via the Apartment gem

Includes:

* Database cleaner config to work correctly with apartment schemas
* Auto-create schemas when a tenant is created
* AdminUsers, Tenant and DelayJob models are "global" across all tenants
* Customize devise mailers to use urls pulled off Tenant#domain
* Interpolate the tenant name into the generated paperclip upload paths
* Add ActiveAdmin tenant config
* Implement a generic elevator that will default to 'public' when no tenant is found
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