Revert "Merge branch 'remove-select2-rails' into 'master'"

This reverts merge request !17871

Gemfile

@@ -267,6 +267,7 @@ gem 'gemojione', '~> 3.3'
 gem 'gon', '~> 6.2'
 gem 'jquery-atwho-rails', '~> 1.3.2'
 gem 'request_store', '~> 1.3'
+gem 'select2-rails', '~> 3.5.9'
 gem 'virtus', '~> 1.0.1'
 gem 'base32', '~> 0.3.0'

Gemfile.lock

@@ -811,6 +811,8 @@ GEM
     seed-fu (2.3.7)
       activerecord (>= 3.1)
       activesupport (>= 3.1)
+    select2-rails (3.5.9.3)
+      thor (~> 0.14)
     selenium-webdriver (3.12.0)
       childprocess (~> 0.5)
       rubyzip (~> 1.2)

app/assets/stylesheets/application.scss

@@ -3,6 +3,7 @@
  * and any sub-directories. You're free to add application-wide styles to this file and they'll appear at
  * the top of the compiled file, but it's generally better to create a new file per style scope.
  *= require jquery.atwho
+ *= require select2
  *= require_self
  *= require cropper.css
 */
@@ -17,7 +18,6 @@
 @import "../../../node_modules/pikaday/scss/pikaday";
 @import "../../../node_modules/dropzone/dist/basic";
-@import "../../../node_modules/select2/select2";
 /*
  * GitLab UI framework