Move the flashbar to the header and add styles for it

 * Add the flashbar to the header so that it shows up
   on all routes that might need it. Otherwise flash
   messages will only appear when the user visits
   the homepage

 * Add styling for the flashbar

h/static/styles/flashbar.scss

+@at-root {
+  .flashbar {
+    max-width: 1130px;
+    margin-left: auto;
+    margin-right: auto;
+  }
+}

h/static/styles/site.scss

@@ -16,6 +16,7 @@
 
 @import './group-form';
 @import './header';
+@import './flashbar';
 @import './footer';
 @import './modal';