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

test/unit/post_test.rb

@@ -695,7 +695,7 @@ class PostTest < ActiveSupport::TestCase
 
         context "that already exists" do
           setup do
-            %W(___ ~foo _foo foo_ foo__bar foo*bar foo,bar foo\abar café 東方).each do |tag|
+            %W(___ ~foo _foo foo_ foo__bar foo*bar foo,bar foo\abar café 東方 new search).each do |tag|
               build(:tag, name: tag).save(validate: false)
             end
           end
@@ -710,6 +710,8 @@ class PostTest < ActiveSupport::TestCase
           assert_invalid_tag("foo\abar")
           assert_invalid_tag("café")
           assert_invalid_tag("東方")
+          assert_invalid_tag("new")
+          assert_invalid_tag("search")
         end
       end