Replace single questions dropdown by plugin

Description

When viewed in dropdown layout of both Single and Single grid question. There are 3 problems in this case:
• Some characters on keyboard decode/encode problem. Such as: Ampersand ( & ), single quotation  ( ' ), double quotation marks ( " ), is more than ( > ), is less than ( < ).
• Same problem with characters not on keyboard. For example: ÷, ×, ±, ≠, ≡, ≤, ≥, …, ‘, °C and maybe more than that.
• Problem with display HTML format in dropdown list when using it to design UI.

Expectation

These decode/encode problems will be fix and display normally like other question type and layout.

Example

Before:

After: File:After-replace.PNG

Solution

Replace select dropdown default by plugin 'select2'.
URL: https://select2.org/

Code

Add this code to Javascript editor in both single or single grid dropdown layout.

```js

Question.bind('afterShowQuestion', function(ev, question, $el) {
 $.when(
  $.fn.select2 || $.ajax({
   type: 'GET',
   dataType: 'script',
   cache: true,
   url: 'https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.13/js/select2.min.js'
  }),
  $.fn.select2 || $('<link/>', {rel: 'stylesheet', href: 'https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.13/css/select2.min.css'}).appendTo('head')
 ).done(function() {
  $('select', $el).select2({
   minimumResultsForSearch: Infinity,
   width: '100%',
   templateResult: function(state) {
    //custom option template in dropdown
    if (!$(state.element).instance()) return null;
    return $('<div/>').html(state.text).css({'text-align': 'left'});
   },
   templateSelection: function(state) {
    //custom selected value
    if (!$(state.element).instance()) return question.selectText;
    return $('<div/>').html(state.text).css({'text-align': 'left'});
   }
  });
  $('.select2-selection', $el).css({height: 'auto', 'min-height': '28px'});
 });
});

```