gui: Use a modern font stack inspired by Bootstrap 4 (#5407)

See https://github.com/twbs/bootstrap/pull/19098 for more information.

cmd/ursrv/static/index.html

@@ -21,7 +21,7 @@ found in the LICENSE file.
   <style type="text/css">
     body {
       margin: 40px;
-      font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
+      font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans", sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji";
     }
     tr.main td {
       font-weight: bold;

gui/default/assets/css/overrides.css

@@ -9,11 +9,11 @@
 
 body {
     padding-bottom: 70px;
-    font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;
+    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans", sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji";
 }
 
 h1, h2, h3, h4, h5 {
-    font-family: "Raleway", "Helvetica Neue", Helvetica, Arial, sans-serif;
+    font-family: "Raleway", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans", sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji";
     line-height: 1.25;
 }