Avoid lego in wikibase-anonymouseditwarning

For logging in, it's not relevant what kind of page we are in.
Actually, why not call it just "page" or use a core message?
For now, switching to generic but technical name; the parameter
is untouched, so languages not bothering about the gender of the
entity kind etc. can use it if they wish.

<https://translatewiki.net/wiki/Thread:Translating_talk:MediaWiki/About_MediaWiki:Wikibase-anonymouseditwarning/en>

Change-Id: Ib1076e9a3cdc944af02eadf6417cdc5bcd4c49aa