Add Whitelabel Support

[madzak]

Overview

With most Saas applications will want to offer custom branding which is where whitelabel support comes into play.

Requirements

The WhiteLabel Feature Shall...

• Allow customers of the Saas application to provide a template file which is stored on the filesystem.
• Based on the tenant that is currently active, the base template will be loaded and injected between the master base and the view templates.
• Users can include external stylesheets, javascript, and images in the custom base template.
• Users base templates should jail'd and not able to access other templates in the Saas system.
• Provide a default base template if one cannot be found

[sleekslush]

We can control the isolation of templates through a custom template loader. Probably one that inherits the behavior of the existing filesystem one.