Update deprecated methods, remove duplicated method, fix typos

[amiedes]

What does this PR do?

• Replaces File::exists? and Dir::exists? deprecated methods in favor of File::exist? and Dir::exist?
  • https://ruby-doc.org/core-2.3.1/File.html#method-c-exists-3F
  • https://ruby-doc.org/core-2.3.1/Dir.html#method-c-exists-3F
• Removes the duplicated site_id method from person_post_form.rb
• Corrects a couple of typos in the documentation

[ferblape]

LGTM!

[danguita]

@amiedes Thanks for that! 👏