Update javascript English translation files and some defaultValue (#1676)

* Reorder javascript English locale file

 * Reorder translation string in order of the locale key.

* Add javascript English locale missing language keys

 * Search all javascript language keys by command:
   `grep -REho '<FormattedMessage .*\/>' ./app/assets/javascripts/.`

 * Add all the missing language keys and their values to `en.jsx`.

* Add javascript English locale missing language keys (2)

* Find all `defineMessages` calls with this command:
  `grep -Rl 'defineMessages({.*' ./app/assets/javascripts/.`

* Open all these files. Find the language key (`id`) in these
  statements.

* Add all the missing language keys and their values to `en.jsx`.

* Remove javascript English locale obsoleted language keys

 * Find all language keys that no longer exists in the source code
  and remove them. The removed keys include:

    * "compose_form.private"
    * "compose_form.unlisted"
    * "getting_started.about_addressing"
    * "getting_started.about_shortcuts"
    * "notification.mention"
    * "search.account"
    * "search.hashtag"
    * "tabs_bar.mentions"
    * "tabs_bar.public"

* Javascript English locale file add note

 * Add notes to contributors about the English translation files.
   Hope that will make translation process smoother.

* Update javascript locale defaultValue in code

 * Update the defaultValue in code according to the relevant
   translation in English locale file.