Remove double underscores from special property message keys

[kghbln]

Setup and configuration

• SMW version: Semantic MediaWiki | 3.0.0-alpha (469a5b7) 09:10, 7 December 2017
• SIL version: Semantic Interlanguage Links | 1.5.0-alpha (e9dcb2a) 04:04, 27 October 2017

Issue

This issue refers to the issue reported at: SemanticMediaWiki/SemanticCite#58 and tackled in SMW core with SemanticMediaWiki/SemanticMediaWiki#2859.

Necessary since MediaWiki merges two consecutive underscores.

[kghbln]

Also noted SemanticMediaWiki/SemanticMediaWiki#1365