AdminUI: Use Select2.js for SiteSetting Lists – adds Autocomplete and DnD-Sorting

[gnunicorn]

This PR rebases the list-sitesetting component on top of Select2.js for better autocompletion and sorting support. For the later it uses the already included html5sorting library. Additionally sitesettings can now also contain a choices-list that is shown as autocomplete (preventing typos) and are automatically merged if extended by plugins. This has been added for post_menu, top_menu and share_links.

To extend this, a plugin just provides a site_settings.yml file with the following:

basic:
  post_menu:
    choices:
      - sharedEdit

Some screenshots:

Autocomplete:

Drag and Drop with placeholder element for positioning:

[discoursebot]

You've signed the CLA, ligthyear. Thank you! This pull request is ready for review.

[riking]

Looks awesome!

[eviltrout]

This is a great PR! Thanks so much.

My only complaint is the modules should be ES6 but we'll get to that later :)

[ZogStriP]

Does it work with values like latest,-Computer Help or category/Computer Help for the top_menu setting?

[eviltrout]

I've removed the choices from the top_menu because of this.

[gnunicorn]

@eviltrout , please don't - cc3119f#commitcomment-6555186 .
@ZogStriP, it allows any input. but right now space, komma and "|" are considered token separators –

discourse/app/assets/javascripts/admin/components/list_setting_component.js

Line 20 in a1887c9

tokenSeparators: [",", " ", "|"],

. We can change that to also include " " if that is officially supported by discourse (which I didn't know).

[ZogStriP]

I think we should remove both space and comma as token separators and we'll be fine.