Add an example on using Flow I18N with Spring

[denis-anisimov]

@pleku commented on Tue Dec 19 2017

Acceptance Criteria

• There is an example of using an I18NProvider bean via Spring in the flow-spring-tutorial
  • pends on #2928
  • just show a label for a localized test from I18NProvider and a button for switching to the other available locale
• There is a section in the flow spring documentation that says that this is possible and people should look at the tutorial code for an example
  • the documentation should only briefly highlight what is needed for this to work

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@heruan commented on Tue Dec 19 2017

As a side note, it would be really great to just be able to expose a I18NProvider bean, instead of setting the servlet parameter.