Add Overleaf header logo

public/stylesheets/components/navbar.less

@@ -386,7 +386,7 @@
     bottom: 5px;
     width: 180px;
     padding: 0;
-    background-image: url('/img/brand/logo-horizontal.svg');
+    background-image: @navbar-brand-image-url;
     background-size: contain;
     background-repeat: no-repeat;
     background-position: left center;

public/stylesheets/core/ol-variables.less

@@ -28,6 +28,8 @@
 @brand-warning:         @orange;
 @brand-danger:          #E03A06;
 
+@navbar-brand-image-url: url(/img/ol-brand/logo-horizontal.png);
+
 @editor-loading-logo-padding-top: 115.44%;
 @editor-loading-logo-background-url: url(/img/ol-brand/overleaf-o-grey.svg);
 @editor-loading-logo-foreground-url: url(/img/ol-brand/overleaf-o.svg);

public/stylesheets/core/variables.less

@@ -25,6 +25,8 @@
 @brand-warning:         @orange;
 @brand-danger:          #E03A06;
 
+@navbar-brand-image-url: url(/img/brand/logo-horizontal.svg);
+
 @editor-loading-logo-padding-top: 86.2%;
 @editor-loading-logo-background-url: url(/img/brand/lion-grey.svg);
 @editor-loading-logo-foreground-url: url(/img/brand/lion.svg);