Rename Notifier to UserMailer

It's a convention in rails to name your mailers with a Mailer suffix, and is also common to name the class after the recipient (e.g. User, Admin). So UserMailer seems a reasonable choice.

app/helpers/user_mailer_helper.rb

@@ -0,0 +1,42 @@
+module UserMailerHelper
+  def fp(text)
+    format_paragraph(text, 72, 0)
+  end
+
+  def link_to_user(display_name)
+    link_to(
+      tag.strong(
+        display_name,
+        # NB we need "text-decoration: none" twice: GMail only honours it on
+        # the <a> but Outlook only on the <strong>
+        :style => "text-decoration: none"
+      ),
+      user_url(display_name, :host => Settings.server_url),
+      :target => "_blank",
+      :rel => "noopener",
+      :style => "text-decoration: none; color: #222"
+    )
+  end
+
+  def message_body(&block)
+    render(
+      :partial => "message_body",
+      :locals => { :body => capture(&block) }
+    )
+  end
+
+  def style_message(html)
+    # Because we can't use stylesheets in HTML emails, we need to inline the
+    # styles. Rather than copy-paste the same string of CSS into every message,
+    # we apply it once here, after the message has been composed.
+    html.gsub(/<p>/, '<p style="color: black; margin: 0.75em 0; font-family: \'Helvetica Neue\', Arial, Sans-Serif">')
+  end
+
+  def style_left
+    I18n.t("html.dir") == "ltr" ? "left" : "right"
+  end
+
+  def style_right
+    I18n.t("html.dir") == "ltr" ? "right" : "left"
+  end
+end