Reduce column-width to 25em for Minerva skin

Current Minerva skin is purposely restricted to 55.8em
This means references will never be split into 2 columns as 30em*2=60em.
Dropping it to 25em would mean the CSS applies here and no more than 2
columns would ever be used to display references.

Bug: T160497
Change-Id: I6f9b85cd6cfdb055692b1f537d569c158900f7c9

extension.json

@@ -54,6 +54,9 @@
 					"media": "print"
 				}
 			},
+			"skinStyles": {
+				"minerva": "ext.cite.styles.minerva.less"
+			},
 			"position": "bottom",
 			"targets": [
 				"desktop",

modules/ext.cite.styles.minerva.less

@@ -0,0 +1,7 @@
+@import "mediawiki.mixins.less";
+
+ .mw-references-columns {
+	/* Minerva skin width is cut to 1000px  (62.5em). With margins/paddings included this
+	leaves 55.8em for content. This ensures that 2 columns will be used where possible */
+	.column-width( 25em );
+}