Rename @fontFamily* variables to WikimediaUI Base naming convention

Renaming:
- @fontFamilySans -> @font-family-system-sans
- @fontFamilyBase -> @font-family-base
As defined in: https://phabricator.wikimedia.org/source/wikimedia-ui-base/browse/master/wikimedia-ui-base.less$201

Bug: T245136
Change-Id: Idec2c516efe2a6ee82efdf3cc6be173bedbb5b56

minerva.less/minerva.variables.less

@@ -12,8 +12,8 @@
  * Note that CSS Fonts Module Level 4's `system-ui` value has resulted in unresolved
  * side-effects in certain OS/language combinations and is therefore not included as of now.
  */
-@fontFamilySans: -apple-system, 'BlinkMacSystemFont', 'Segoe UI', 'Roboto', 'Lato', 'Helvetica', 'Arial', sans-serif;
-@fontFamilyBase: @fontFamilySans;
+@font-family-system-sans: -apple-system, 'BlinkMacSystemFont', 'Segoe UI', 'Roboto', 'Lato', 'Helvetica', 'Arial', sans-serif;
+@font-family-base: @font-family-system-sans;
 
 @font-size-browser: 16; // Assumed browser default of `16px`.
 @font-size-root: 100%;

resources/skins.minerva.content.styles/main.less

@@ -7,7 +7,7 @@
 // Our content is predominately text, hence visual hierarchy must be clear.
 
 body {
-	font-family: @fontFamilyBase;
+	font-family: @font-family-base;
 	line-height: 1.4;
 	-webkit-tap-highlight-color: rgba( 0, 0, 0, 0.2 );
 	overflow-y: scroll;

resources/skins.minerva.content.styles/toc.less

@@ -38,7 +38,7 @@
 
 		h2 {
 			display: inline-block;
-			font-family: @fontFamilyBase;
+			font-family: @font-family-base;
 			font-size: unit( 18/16, em );
 			vertical-align: middle;
 		}

resources/skins.minerva.talk.styles/talkpage.less

@@ -54,7 +54,7 @@
 		position: relative;
 		border-bottom: 1px solid @border-color-divider;
 		font-size: 1em;
-		font-family: @fontFamilyBase;
+		font-family: @font-family-base;
 		line-height: 1;
 		cursor: pointer;
 		display: block;

skinStyles/mobile.special.styles/minerva.less

@@ -31,7 +31,7 @@
 	h1,
 	h2 {
 		// Important given we have no idea which rules special pages are enforcing
-		font-family: @fontFamilyBase !important;
+		font-family: @font-family-base !important;
 	}
 
 	.content-header {