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.rb

@@ -0,0 +1,39 @@
+class PostNavbarComponent < ApplicationComponent
+  extend Memoist
+
+  attr_reader :post, :current_user, :search, :pool_id, :favgroup_id
+
+  def initialize(post:, current_user:, search: nil, pool_id: nil, favgroup_id: nil)
+    @post = post
+    @current_user = current_user
+    @search = search.presence || "status:any"
+    @pool_id = pool_id&.to_i
+    @favgroup_id = favgroup_id&.to_i
+  end
+
+  def render?
+    has_search_navbar? || pools.any? || favgroups.any?
+  end
+
+  def pools
+    post.pools.undeleted.sort_by do |pool|
+      [pool.id == pool_id ? 0 : 1, pool.is_series? ? 0 : 1, pool.name]
+    end
+  end
+
+  def favgroups
+    current_user.favorite_groups.for_post(post.id).sort_by do |favgroup|
+      [favgroup.id == favgroup_id ? 0 : 1, favgroup.name]
+    end
+  end
+
+  def has_search_navbar?
+    !has_order_metatag? && pool_id.blank? && favgroup_id.blank?
+  end
+
+  def has_order_metatag?
+    PostQueryBuilder.new(search).has_metatag?(:order, :ordfav, :ordpool)
+  end
+
+  memoize :pools, :favgroups
+end