HTML e-mails for UserMailer (#6256)

- premailer gem to turn CSS into inline styles automatically
- rework UserMailer templates
- reword UserMailer templates

lib/mastodon/premailer_webpack_strategy.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module PremailerWebpackStrategy
+  def load(url)
+    if Webpacker.dev_server.running?
+      url = File.join("#{Webpacker.dev_server.protocol}://#{Webpacker.dev_server.host_with_port}", url)
+      HTTP.get(url).to_s
+    else
+      url = url[1..-1] if url.start_with?('/')
+      File.read(Rails.root.join('public', url))
+    end
+  end
+
+  module_function :load
+end