Fallback io (Ido) to eo (Esperanto)

Fallback io (Ido) to eo (Esperanto)

Ido is a variant of Esperanto and all its speakers are supposed
to also know Esperanto. Either way, all translators were notified.

https://translatewiki.net/wiki/Thread:Portal_talk:Ido/Use_Esperanto_when_no_translation

From eo, io will inherit the date format, usage of space as digit separator
(consistent with most European languages), hundreds of translations for
magic words and special pages. The namespace gender alias for eo is adapted
to match the io format.

All the io.wiki and io.wiktionary users as listed by WikiStats have been
notified of the discussion individually.

Change-Id: Ia85d2e4b007b523027891bf2c2bccde55916510c