Use HTTP Accept-Language to detect locale (#1166)

* Use HTTP Accept-Language to detect locale

* Fix gem order to comply with codeclimate

* Sort gem to comply with rubocop

* I18n.default_locale fallback when there is no accept-language header

Gemfile.lock

@@ -184,6 +184,7 @@ GEM
     http-cookie (1.0.3)
       domain_name (~> 0.5)
     http-form_data (1.0.1)
+    http_accept_language (2.1.0)
     http_parser.rb (0.6.0)
     httplog (0.3.2)
       colorize
@@ -473,6 +474,7 @@ DEPENDENCIES
   hiredis
   htmlentities
   http
+  http_accept_language
   httplog
   i18n-tasks (~> 0.9.6)
   jquery-rails
@@ -522,4 +524,4 @@ RUBY VERSION
    ruby 2.3.1p112
 
 BUNDLED WITH
-   1.14.3
+   1.14.5