overload jingo.helpers.f to fix UnicodeEncodeError (bug 976727)

This PR is only adding one change to the commits in PR #1740 (fixing bug 963513): it adds a ```|f``` helper in ```amo.helpers``` to overload jingo's one.

The issue is explained here: https://github.com/jbalogh/jingo/pull/54#issuecomment-36728948

We thus first check if the input of the ```|f``` helper is an instance of
```six.text_type```, and if not, we convert before doing the string
interpolation.