[gnome-shell] String additions

[Rafal Luzynski <digitalfreak lingonborough com>]

As we are in the String Change Announcement Period here I announce
that the new strings have been added to GNOME Shell:

+ "%OB"
+ "%OB %Y"

Translators: these are date format strings and probably in most of
the languages you should leave them unchanged. Unless you want to
display the year number before the month name.

Hint: Spanish (and similar languages) translators may prefer the
date format "%OB de %Y" (to display something like "febrero de 2018").

Thank you Florian for reviewing accepting this change.

Regards,

Rafal