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/models/wiki_page.rb

@@ -23,6 +23,14 @@ class WikiPage < ApplicationRecord
   api_attributes including: [:category_name]
 
   module SearchMethods
+    def find_by_id_or_title(id)
+      if id =~ /\A\d+\z/
+        [find_by_id(id), :id]
+      else
+        [find_by_title(normalize_title(id)), :title]
+      end
+    end
+
     def titled(title)
       where(title: normalize_title(title))
     end
@@ -138,7 +146,7 @@ class WikiPage < ApplicationRecord
   end
 
   def self.normalize_title(title)
-    title.downcase.gsub(/[[:space:]]+/, "_").gsub(/__/, "_").gsub(/\A_|_\z/, "")
+    title.downcase.delete_prefix("~").gsub(/[[:space:]]+/, "_").gsub(/__/, "_").gsub(/\A_|_\z/, "")
   end
 
   def normalize_title
@@ -235,4 +243,12 @@ class WikiPage < ApplicationRecord
   def visible?
     artist.blank? || !artist.is_banned? || CurrentUser.is_builder?
   end
+
+  def to_param
+    if title =~ /\A\d+\z/
+      "~#{title}"
+    else
+      title
+    end
+  end
 end