posts: factor out post navbar into component.

* Factor out the post navbar into a component. The post navbar is the
  part of the post containing the current search, the list of pools, and
  the list of favgroups, along with next/prev navigation links.

* Change navbar markup: remove various unused CSS classes/IDs, change
  pools to use same markup as favgroups, replace nested <div>'s with
  flat <ul>/<li> list.

* Use CSS to truncate long searches/pool names/favgroup names if they're
  too wide for the screen (especially on mobile).

app/components/post_navbar_component/post_navbar_component.scss

@@ -0,0 +1,31 @@
+.post-notice-search {
+  li {
+    display: flex;
+
+    &[data-selected="true"] {
+      font-weight: bold;
+    }
+
+    .first, .prev, .next, .last {
+      flex: 0;
+      margin: 0 0.25em;
+      white-space: nowrap;
+    }
+
+    .first, .last {
+      flex-basis: 1em;
+    }
+
+    &.search-navbar .prev, &.search-navbar .next {
+      margin: 0 1.75em;
+    }
+
+    .pool-name, .favgroup-name, .search-name {
+      flex: 1;
+      text-align: center;
+      overflow: hidden;
+      white-space: nowrap;
+      text-overflow: ellipsis;
+    }
+  }
+}