Link to existing login help page by default from helplogin-url

Detailed rationale discussed on bugzilla, bug 53888 and parent,
where consensus on the design was gained.

Using https link per Brion et al. in comments and bug 54835,
with new keys for the messages in order to avoid any conflict
with existing messages. (The translations of the removed messages
will be automatically removed by localisation updates.)

https://www.mediawiki.org/wiki/Help:Logging_in was improved by Lloffiwr
and now also marked for translation. The link is ready for future
translations thanks to the use of the standard Special:MyLanguage, hence
the message is made ignored to allow customisations only locally.

Bug: 53888
Change-Id: I8cefafbcd3bd1d6a03cec7cf1d0efdf607fec21d