[fix] [notifications] fix icon width

When notifications are present, the wrong icon width was used. Set the
icon width and remove a useless class, main-header-button.

https://codesearch.wmflabs.org/search/?q=main-header-button&i=nope&files=&repos=

Change-Id: Ie274ffcc93cf576f36eb0fccee80ecd3d4ac3a21

includes/skins/userNotifications.mustache

@@ -1,6 +1,6 @@
 {{^hasNotifications}}
 <a href="{{url}}" title="{{title}}"
-   class="{{notificationIconClass}} user-button main-header-button" id="user-notifications">
+   class="{{notificationIconClass}} user-button" id="user-notifications">
     {{title}}
 </a>
 {{/hasNotifications}}

resources/skins.minerva.loggedin.styles/styles.less

@@ -14,6 +14,10 @@
 	}
 }
 
+.minerva-user-notifications {
+	width: @menuButtonIconSize;
+}
+
 .notification-count {
 	@circleSize: 24px;
 	@borderSize: 2px;