Feature: #104020 - ViewHelper to check feature flags 

See forge#104020

Description 

The <f:feature> ViewHelper allows integrators to check for feature flags from within Fluid templates. The ViewHelper follows the same rules as the underlying TYPO3 api, which means that undefined flags will be treated as false.

Examples 

Basic usage 

<f:feature name="myFeatureFlag">
   This is being shown if the flag is enabled
</f:feature>
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feature / then / else 

<f:feature name="myFeatureFlag">
   <f:then>
      Flag is enabled
   </f:then>
   <f:else>
      Flag is undefined or not enabled
   </f:else>
</f:feature>
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Impact 

Feature flags can now be checked from within Fluid templates.