Explain reset token expiration in emails

Update the new user emails to tell new users when their password reset token expires and provide a link to get a new one.  See #1921.  This MR adds new text to the emails:

```html
This link is valid for X days.  After it expires, you can <a href='new_user_password_url'>request a new one</a>
```

It will be more difficult to add the same link to the error message that's displayed when a user tries to reset his password with an expired token.  Currently, we don't know why the password change fails, we just show the Devise error messages on the form.

See merge request !1803