Add Languages::resetFallbackPageExtensions (#1276)
Add Languages::resetFallbackPageExtensions (#1276)

Resets the page_extensions value.
Useful to re-initialize the pages and change site language at runtime,
example:
```
$this->grav['language']->setActive('it');
$this->grav['language']->resetFallbackPageExtensions();
$this->grav['pages']->init();
```
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