Move admin header partial to a component

This way it's easier to refactor it.

Note we're using `with_request_url` in the tests because the component
renders the locale switcher, which needs a URL in order to work. This
doesn't affect whether we're in the management section or not.

app/assets/stylesheets/admin.scss

@@ -31,14 +31,6 @@ $table-header:   #ecf1f6;
 .admin {
   @include admin-layout;
 
-  > header {
-    border-bottom: 1px solid #eee;
-
-    > * {
-      @include full-width-background($adjust-margin: false, $adjust-padding: true);
-    }
-  }
-
   h2 {
     font-weight: 100;
     margin-bottom: $line-height;
@@ -60,86 +52,6 @@ $table-header:   #ecf1f6;
     float: none;
   }
 
-  .top-links {
-
-    a {
-      line-height: rem-calc($line-height * 1.5);
-    }
-  }
-
-  .top-bar {
-    height: auto;
-    padding-top: $line-height / 2;
-
-    @include breakpoint(small only) {
-
-      .top-bar-left ul {
-        display: inline-block;
-      }
-
-      .top-bar-right {
-
-        > ul {
-          border-bottom: 0;
-          padding-bottom: 0;
-          margin-bottom: 0;
-        }
-
-        .submenu {
-          position: initial;
-        }
-
-        a {
-          font-weight: normal;
-        }
-      }
-
-      [class^="icon-"] {
-        display: none;
-      }
-    }
-
-    [class^="icon-"] {
-      font-size: $base-font-size;
-    }
-
-    h1 {
-      margin-top: $line-height / 2;
-      margin-bottom: 0;
-
-      @include breakpoint(medium) {
-        margin-top: 0;
-      }
-
-      small {
-        color: inherit;
-        text-transform: uppercase;
-      }
-
-      a {
-        color: inherit;
-        display: inline-block;
-        font-family: "Lato" !important;
-        font-size: rem-calc(24);
-        font-weight: lighter;
-        line-height: 1;
-      }
-    }
-  }
-
-  .top-bar .menu > li {
-
-    @include breakpoint(medium) {
-      height: auto !important;
-    }
-  }
-
-  .title-bar {
-    color: inherit;
-    position: absolute;
-    right: 12px;
-  }
-
   .notifications.unread-notifications::after {
     color: $admin-color;
   }