minor #384 Use .xlf extension instead of .xliff one for translations (bocharsky-bw)

This PR was merged into the master branch.

Discussion
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Use .xlf extension instead of .xliff one for translations

Symfony uses `.xlf` extension in its [XliffFileDumper](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Translation/Dumper/XliffFileDumper.php#L53-L56) instead of `.xliff` which currently uses by Symfony Demo. BTW, the `.xlf` is correct filename extension according to the [wiki](https://en.wikipedia.org/wiki/XLIFF) page about XLIFF format.

What about to change translations' extension for consistency?

Commits
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b760de8 Use .xlf extension instead of .xliff one for translations

Author: javiereguiluz