Add shared layout view

- change all onebox engine templates from `*.handlebars` to `*.mustache`
- sticking with Mustache since Handlebars has no good support for Ruby
- onebox engine templates each render within `_layout.mustache` template as partials
- decouples template rendering from `Engine` module as a new `View` class
- all onebox engines must have Mustache template
- remove all duplicated elements from onebox engine templates