Remove limited content from email notifications

This is my solution to pull request #4508 that solves issue #4342 and #4266 

I have doubts in messages 'notifier.a_private_message' and 'notifier.a_limited_post_comment' due to I'm not a native English speaker.

I also updated 'spec/mailers/notifier_spec.rb' file 'cause now if post is public or not changes email text, but I don't know if this file requires new tests for private mails. What do you think?