wikis: force wiki names to follow same rules as tag names.

Don't allow wiki pages to have invalid names.

This incidentally means that you can't create wiki pages for pools. For
example, you can't create a wiki titled "pool:almost_heart-warming".
This is not a valid tag name, so it's not a valid wiki name either. This
was done in a handful of cases to translate Pixiv tags to Danbooru pools
(see: <https://danbooru.donmai.us/wiki_page_versions?search[title_like]=pool:*>)

Also fix it so that titles are normalized before validation, not before save.

test/functional/wiki_pages_controller_test.rb

@@ -109,13 +109,6 @@ class WikiPagesControllerTest < ActionDispatch::IntegrationTest
         assert_response 404
       end
 
-      should "render for a negated tag" do
-        as(@user) { @wiki_page.update(title: "-aaa") }
-
-        get wiki_page_path(@wiki_page.id)
-        assert_redirected_to wiki_page_path(@wiki_page.title)
-      end
-
       should "work for a title containing dots" do
         as(@user) { create(:wiki_page, title: "...") }