Migrate away from select2 in ee/app/assets/javascripts/pages/admin/application_settings/advanced_search/index.js

The ee/app/assets/javascripts/pages/admin/application_settings/advanced_search/index.js script initializes select2 instances on the .js-elasticsearch-namespaces and .js-elasticsearch-projects selectors, which are being used in ee/app/views/admin/application_settings/_elasticsearch_form.html.haml.

Field	Preview
Admin area > Settings > Advanced Search > Namespaces to index
Admin area > Settings > Advanced Search > Project to index

Both instances provide server-side filtering and single selection.

As part of this issue, we'd like to migrate away from select2 in favor of GlTokenSelector or any other Pajamas-compliant component that would be appropriate.

Implementation plan

• Migrate the Admin area > Settings > Advanced Search > Namespaces to index field away from select2.
• Migrate the Admin area > Settings > Advanced Search > Project to index field away from select2.
• Remove select2-related code from ee/app/assets/javascripts/pages/admin/application_settings/advanced_search/index.js.