Display month names correctly according to the new standard

This pull request contains 2 patches which fix 2 bug reports from the old GNOME Bugzilla:

1. https://bugzilla.gnome.org/show_bug.cgi?id=781329 - Make month and month-year formats translatable. Instead of retrieving the month-year order from GTK+ and building either "%B %Y" or "%Y %B" it makes the string translatable and leaves the decision to the translators. The translators will have not only a choice whether to use "%B %Y" or "%Y %B" but there may be more complex choices, for example Spanish translators may want the format specifier "%B de %Y" (so they will get a string like "febrero de 2018").
2. https://bugzilla.gnome.org/show_bug.cgi?id=780957 - Preparing gnome-shell for genitive/nominative month names. Since glibc 2.27 (released February 1, 2018) there are new format specifier and the correct way to display the month standalone or with the year number only is to use "%OB" rather than "%B". Note that this feature works only if glibc is 2.27 or newer (or if the underlying system is BSD or OS X).