ikiwiki's PO plugin doesn't include inline in language stats

Originally created by @sajolida on #16867 (Redmine)

For example, right now:

https://tails.boum.org/install/win/usb/

is advertised as 100% translated in Italian while it’s not. It’s more like 10% translated.

I guess that’s because what counts in the stats is only the strings of wiki/src/install/win/usb.it.po and not the strings in wiki/src/install/inc/steps/install_with_etcher.inline.it.po for example.

An algorithm to compute better stats could be:

• If an inline directive is translated in the parent PO file and corresponds to a translated inline, count the strings in the inline PO file. Almost all inlines have translations but not all (eg. @wiki/src/inc).
• Otherwise, count the inline directive itself.

Feature Branch: web/16867-remove-percent-translated