* adds select2 placeholders to select lists
To allow us to clear the selection on „select2“ selects, we need a placeholder attribute
See: https://select2.org/placeholders
* Removes empty option from multiple select
select2 requires an empty option value on singular selects, but not on multiple selects.
When selecting multiple options, this empty option would be shown as selectable otherwise, not clearing the selection.
* Adds the option to clear select2 instances
Sets the correct options to allow clearing of out select2 instances. The empty placeholder is required, since clearing only works when a placeholder ist set (event an empty one).
See: https://select2.org/placeholders
* Removes the „Clear selection“ option from select lists
Since we can clear the select2 lists with their native clearing method, we can remove this hack
* Updates generated assets (css/js)
