Add link to PD help translatable pages from sidebar

Reuses the existing helppage default message and
adds it to ignored messages as Special:MyLanguage
is used and translations have to be removed:
this does not bother customised local links.

Now using HTTPS URL to mediawiki.org per Brion
on bug 54835. This slightly increases the footprint
of the change but should keep everything functionally
equivalent. The sidebar is not bothered by full URLs,
except in self-defeating tests which are also fixed here.

Bug: 53887
Change-Id: I999b97729536dbab4a3a5efd8d6f86527f031948