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/helpers/wiki_pages_helper.rb

@@ -1,7 +1,7 @@
 module WikiPagesHelper
   def wiki_page_excerpt(wiki_page)
     text = strip_dtext(wiki_page.body).split(/\r\n|\r|\n/).first
-    truncate(text, length: 160)
+    text.truncate(160)
   end
 
   def wiki_page_other_names_list(wiki_page)