wiki pages: use names instead of ids in urls.

Switching to using wiki names in URLs instead of IDs:

* https://danbooru.donami.us/wiki_pages/vocaloid
* https://danbooru.donami.us/wiki_pages/hatsune_miku

ID numbers can still be used, but they redirect to the name instead:

* https://danbooru.donami.us/wiki_pages/11 (redirects to /wiki_pages/touhou).

Numeric tags are prefixed with '~' to distinguish them from IDs:

* https://danbooru.donami.us/wiki_pages/2019 (the wiki with id 2019)
* https://danbooru.donami.us/wiki_pages/~2019 (the wiki for the tag named 2019)

The tag names 'new' and 'search' are disallowed to prevent conflicts
with existing routes:

* https://danbooru.donami.us/wiki_pages/new
* https://danbooru.donami.us/wiki_pages/search

app/helpers/application_helper.rb

@@ -103,13 +103,14 @@ module ApplicationHelper
     link_to search, posts_path(tags: search)
   end
 
-  def link_to_wiki(text, title)
-    link_to text, wiki_page_path(title)
+  def link_to_wiki(text, title = text, **options)
+    title = "~#{title}" if title =~ /\A\d+\z/
+    link_to text, wiki_page_path(title), class: "wiki-link", **options
   end
 
   def link_to_wikis(*wiki_titles, last_word_connector: ", or", **options)
     links = wiki_titles.map do |title|
-      link_to title.tr("_", " "), wiki_page_path(title)
+      link_to_wiki title.tr("_", " "), title
     end
 
     to_sentence(links, **options)