Avoid using the same "DisqusShortname" config setting as DisqusTag

Authored by Legoktm on Tue, May 15, 6:06 PM.

Description

Avoid using the same "DisqusShortname" config setting as DisqusTag

As of MediaWiki 1.31, extensions are required to have unique configuration
settings to avoid clobbering each other. In this case, there is value
in sharing the Disqus short name between extensions, so first look for
the new $wgSectionDisqusShortname (defaults to false), otherwise fall
back to the original $wgDisqusShortname if it's set, and then a hardcoded
placeholder value that is the old default.

Bug: T194405
Change-Id: I3095c83a861adea8f8d36c89d1250cd1755f8b8d
(cherry picked from commit f49c54c747d0557330b596167e6f47bcac4fba98)