Remove support for legacy IE - #5834

* Replace usage of $.trim with String.prototype.trim

This officially drops support for Internet Explorer 8, since this
method is not provided by that browser. This is one more step closer
to removing the dependency on jQuery itself.

* Replace $.isArray with Array.isArray

Now that we no longer support IE8, we can rely on this method
actually existing now.

* Remove old getAttribute workaround for IE7

This has long been unsupported, and now that we don't care about
IE8 with quirks mode, we can go back to expecting that `getAttribute`
actually returns the string like it is supposed to.

* Remove jQuery from Utils.copyNonInternalCssClasses

This also simplifies the logic used to use more modern array methods
to calculate the list of classes which need to be copied over to
the destination object.

* Remove unused imports

* Remove use of jQuery.fn.is()

* Replace $.inArray with Array.prototype.indexOf

* Replace addClass, hasClass, removeClass with classList calls

This replaces calls to the jQuery `getClass`, `hasClass`, and
`removeClass` methods with the corresponding calls to `classList`
methods on the elements that are referenced. There is one exception
to this, specifically where results remove a class from any elements
which may contain it, since that cannot be easily translated to use
the `classList` syntax.

* Remove legacy DOM modification tracking

This removes the legacy tracking for `onpropertychange`, which was
used by IE 8, to synchronize attribute changes from the `<select>`
element back to Select2.

This removes the legacy tracking of `DOMNodeInserted`,
`DOMNodeRemoved`, and `DOMAttrModified` which was used by IE 9 and
IE 10 for tracking when `<option>` elements were added and removed,
as well as when attributes on the `<select>` element were
synchronized.

Now only the `MutationObserver` is in use for synchronizing changes
to the `<select>` as well as changes to the `<option>` elements that
it contains.

* Replace $.map with Array.prototype.map