Select2 will now automatically update the selection if there are options
added to or removed from the DOM within the `<select>` element. This is
supported in all browsers except for Internet Explorer 8. Internet
Explorer 8 does not support the DOM mutation events which were added in
Internet Explorer 9, and it does not support mutation observers which
are the recommended way of handling this in modern browsers.
DOM mutation events also trigger for the `<select>` itself when it is
pulled from the DOM, so we need to filter these out within the event
handler.
Despite supporting mutation observers, we cannot accurately detect if
the removed option was selected at one time or another, so we need to
always re-pull the selection when an element is deleted.
This closes https://github.com/select2/select2/issues/4248
This builds upon https://github.com/select2/select2/pull/4249
ea79a197e0