i18n regression for strings with variables

Right now in fractal-next we only use the gettext-rs crate.

When we need to use strings with variables. gettext-rs provides macros that allow to use strings like {} is a {}. The issue is that the variables cannot be named so if the translator needs to change the order in the translation they cannot, the first variable will always have the first value and so on.

There is some ongoing discussion about improving the macros at gettext-rs#86.

In the meantime we could reuse the i18n module of fractal-legacy. We should remove the _f methods in this case, otherwise we could still have the same issue.