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/logical/post_sets/post.rb

@@ -17,7 +17,7 @@ module PostSets
     end
 
     def humanized_tag_string
-      tag_array.slice(0, 25).join(" ").tr("_", " ")
+      tag_array.map { |tag| tag.tr("_", " ").titleize }.to_sentence
     end
 
     def has_blank_wiki?