posts: normalize Unicode to NFC form in post sources.

Fix strings like "pokémon" (NFD form) and "pokémon" (NFC form) being
considered different strings in sources.

Also add a fix script to fix existing sources. There were only 15 posts
with unnormalized sources.

script/fixes/097_normalize_post_sources.rb

@@ -0,0 +1,14 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+with_confirmation do
+  CurrentUser.scoped(User.system, "127.0.0.1") do
+    Post.where("source ~ '[^[:ascii:]]'").find_each do |post|
+      next if post.source.unicode_normalize(:nfc) == post.source
+
+      post.update!(source: post.source)
+      puts({ id: post.id, old_source: post.source_before_last_save, new_source: post.source })
+    end
+  end
+end