HTML e-mails for UserMailer (#6256)

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

spec/mailers/previews/user_mailer_preview.rb

@@ -0,0 +1,32 @@
+# Preview all emails at http://localhost:3000/rails/mailers/user_mailer
+
+class UserMailerPreview < ActionMailer::Preview
+  # Preview this email at http://localhost:3000/rails/mailers/user_mailer/confirmation_instructions
+  def confirmation_instructions
+    UserMailer.confirmation_instructions(User.first, 'spec')
+  end
+
+  # Preview this email at http://localhost:3000/rails/mailers/user_mailer/email_changed
+  def email_changed
+    user = User.first
+    user.unconfirmed_email = 'foo@bar.com'
+    UserMailer.email_changed(user)
+  end
+
+  # Preview this email at http://localhost:3000/rails/mailers/user_mailer/password_change
+  def password_change
+    UserMailer.password_change(User.first)
+  end
+
+  # Preview this email at http://localhost:3000/rails/mailers/user_mailer/reconfirmation_instructions
+  def reconfirmation_instructions
+    user = User.first
+    user.unconfirmed_email = 'foo@bar.com'
+    UserMailer.confirmation_instructions(user, 'spec')
+  end
+
+  # Preview this email at http://localhost:3000/rails/mailers/user_mailer/reset_password_instructions
+  def reset_password_instructions
+    UserMailer.reset_password_instructions(User.first, 'spec')
+  end
+end