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