Infer version 915695

This changes the firefox os detection code to also infer the firefox os platform and browser version from the browser parts of the user agent.

There's some data in production that has "wrong gecko versions", so the migration ignores those figuring I don't know what it is if it doesn't have a correct Gecko version.

To test, run the tests, run the migration forwards and backwards and test the web form with FirefoxOS browser.

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