emails: fix sending emails to invalid addresses.

Fix mailers to not attempt deliveries to invalid or nonexistent email
addresses. This usually happened when someone changed their email, and
we tried to send a confirmation email to a nonexistent address.

app/controllers/users_controller.rb

@@ -90,7 +90,7 @@ class UsersController < ApplicationController
       flash[:notice] = "Sign up failed: #{@user.errors.full_messages.join("; ")}"
     else
       session[:user_id] = @user.id
-      UserMailer.welcome_user(@user).deliver_later if @user.can_receive_email?(require_verification: false)
+      UserMailer.welcome_user(@user).deliver_later
       set_current_user
     end