WebActivator fix: concurrent requests (#492).

The Thymeleaf Context object (the view model) is created in **request-scope** (instead of WebActivator-global).
This allows for
    a) concurrent requests to the same web application.
    b) deployment of more than one web application plugin at the same time.

BREAKING CHANGES

1. Your web application plugin's init() hook must call `initTemplateEngine()`.
Formerly this was named `setupRenderContext()`.

2. For page preparation call `setViewModel(name, value)`.
Formerly you did `context.setVariable(name, value)`.

3. Keep in mind a new Thymeleaf Context object is created with each request. For page preparation your plugin could still rely on data kept in the (authenticated) user session. You just need to transfer the required session data to the context (by calling setViewModel()) when preparing the page.

See #492.