Upgrade select2 to stable version 4.0.4 and fix compatibility issues in Civi

Currently, Civi is using select2 3.5 which we need to upgrade to latest stable 4.0.4 (https://github.com/select2/select2/releases) and also resolve compatibility issues with CiviCRM.

Major issues:
1. Auto appending values doesn't work anymore
Error: Uncaught TypeError: Cannot read property 'prop' of undefined
LOC - https://github.com/civicrm/civicrm-core/blob/master/js/crm.searchForm.js#L9
Reference - https://select2.org/upgrading/migrating-from-35#select2-val

2. Placeholder broken
Error: Uncaught TypeError: Cannot read property 'id' of undefined
As per 3.5 - placeholderOption is used to set the placeholder value for select2 widget
As per 4.0.4 - placeholder option can now accept placeholder value both in string and object
Reference - https://select2.org/upgrading/migrating-from-35#more-flexible-placeholders

3. EntityRef select2 fields are broken
Error: Uncaught Error: No select2/compat/initSelection
As per 4.0.4 - Removed the requirement of 'initSelection'
Reference - https://select2.org/upgrading/migrating-from-35#removed-the-requirement-of-initselection

4. Navigation menu style is not applied
Error: Uncaught TypeError: Cannot read property 'prop' of undefined
As per 4.0.4 - $("select").val("1").trigger("change"); // instead of $("select").select2("val", "1");
Reference - https://select2.org/upgrading/migrating-from-35#select2-val

Minor issues
1. Multi select2 widget doesn't show downarrow placeholder icon
2. select2 css style aren't applied after upgrade