Add Estonian language translation

[rakvium]

• add et.yml locale file to config/locales
• copy https://github.com/rakvium/blog/blob/master/config/locales/en.yml content into it
• replace English translations in et.yml file with Estonian ones
• add et into array of available locales in https://github.com/rakvium/blog/blob/master/config/application.rb
• submit a pull request with your changes
• get the tests pass

Notice: language_name string would be for Estonian language name in Estonian

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More on localization for Ruby on Rails: https://guides.rubyonrails.org/i18n.html
Key words: Eesti keel, Estonian, i18n, internationalisation, internationalization, localisation, localization