Adding arabic string localization (rebased) (#1895)

* Create ar.jsx

* Preparing file for translation.

* Create ar.yml

* Preparing file for translation.

* Create simple_form.ar.yml

* Preparing file for translation.

* Create devise.ar.yml

* Preparing file for translation.

* Create doorkeeper.ar.yml

* Preparing file for translation.

* Update index.jsx

Preparing file for translation.

* Update mastodon.jsx

* Preparing file to translation.
* Correction thanks to @Taekiro
  https://github.com/tootsuite/mastodon/pull/1798#pullrequestreview-32924922

* Update settings_helper.rb

Preparing file to translation.

* Update application.rb

* Preparing file to translation.

* Update simple_form.ar.yml

* Update doorkeeper.ar.yml

* Update devise.ar.yml

* Update ar.yml

* Fix i18n-tasks unused issue (ar)

Remove unused translation strings

* Update application.rb

* Update ar.jsx

* Fix various issues with Arabic translation

* Fix duplication caused in merge
* Fix syntax error in devise.ar.yml
* Fix import error Duplicated incorrect import in mastodon.jsx
* Fix duplication error in locales/index.jsx

* Update ar.yml

* Update ar.jsx

* Update ar.yml

* Update ar.jsx

* Fix ar translation