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).

test/functional/posts_controller_test.rb

@@ -556,7 +556,7 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
           get post_path(@post)
 
           assert_response :success
-          assert_select "#pool-nav .pool-name", /Pool: comic/
+          assert_select ".pool-navbar .pool-name", /Pool: comic/
         end
       end