Replace Selectize with Tom Select

The Selectize library is not maintained for some years and its last version includes serious regressions that make it unusable. I looked for an alternative and I found Tom Selec that started as a fork of Selectize, and it seems to be maintained. The good part is that it maintains the Selectize JavaScript API, helping with backwards compatibility. See https://forum.xwiki.org/t/replacing-selectize-with-tom-select/18252 for more details.