autocomplete: render html server-side.

Render the HTML for autocomplete results server-side instead of in
Javascript. This is cleaner than building HTML in Javascript, but it may
hurt caching because the HTTP responses are larger.

Fixes #4698: user autocomplete contains links to /posts

Also fixes a bug where tag counts in the autocomplete menu were different
from tag counts displayed elsewhere because of differences in rounding.

app/components/autocomplete_component.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class AutocompleteComponent < ApplicationComponent
+  attr_reader :autocomplete_service
+
+  delegate :humanized_number, to: :helpers
+  delegate :autocomplete_results, to: :autocomplete_service
+
+  def initialize(autocomplete_service:)
+    @autocomplete_service = autocomplete_service
+  end
+
+  def link_to_result(result, &block)
+    case result.type
+    when "user"
+      link_to user_path(result.id), class: "user-#{result.level}", "@click.prevent": "", &block
+    when "pool"
+      link_to pool_path(result.id), class: "pool-category-#{result.category}", "@click.prevent": "", &block
+    else
+      link_to posts_path(tags: result.value), class: "tag-type-#{result.category}", "@click.prevent": "", &block
+    end
+  end
+end

app/components/autocomplete_component/autocomplete_component.html.erb

@@ -0,0 +1,20 @@
+<ul>
+  <% autocomplete_results.each do |result| %>
+    <%= tag.li class: "ui-menu-item", "data-autocomplete-type": result.type, "data-autocomplete-antecedent": result.antecedent, "data-autocomplete-value": result.value, "data-autocomplete-category": result.category, "data-autocomplete-post-count": result.post_count do %>
+      <div class="ui-menu-item-wrapper" tabindex="-1">
+        <%= link_to_result result do %>
+          <% if result.antecedent.present? %>
+            <span class="autocomplete-antecedent"><%= result.antecedent.tr("_", " ") %></span>
+            <span class="autocomplete-arrow">→</span>
+          <% end %>
+
+          <%= result.label %>
+        <% end %>
+
+        <% if result.post_count %>
+          <%= tag.span humanized_number(result.post_count), class: "post-count", style: "float: right;" %>
+        <% end %>
+      </div>
+    <% end %>
+  <% end %>
+</ul>