views: update seo title and description of post index page.

Change the title of the post index page to look like this:

    "Danbooru: Anime Image Board" (for the front page)
    "Kantai Collection Art | Danbooru" (for a tag search)

Change the meta description of the front page to look like this:

    Danbooru is the original anime image 'booru. Find over 3.75 million
    anime pictures categorized by over 100 million tags.

Change the meta description for a tag search to look like this:

    Find over 37,168 Azur Lane images on Danbooru. Azur Lane (碧蓝航线)
    (벽람항로) is a Chinese shipgirl-themed side-scrolling shoot 'em up
    mobile game developed by Shanghai Manjuu and Xiamen Yongshi...

app/presenters/post_presenter.rb

@@ -160,10 +160,6 @@ class PostPresenter < Presenter
     PostPresenter.preview(@post)
   end
 
-  def humanized_tag_string
-    @post.tag_string.split(/ /).slice(0, 25).join(", ").tr("_", " ")
-  end
-
   def humanized_essential_tag_string
     @humanized_essential_tag_string ||= tag_set_presenter.humanized_essential_tag_string(default: "##{@post.id}")
   end