Admin UI v2

# Description

as per [our roadmap](https://github.com/alfio-event/alf.io/issues/435), we want to rewrite from scratch the admin UI during 2.0-M5.
This will remove a lot of technical debt and will let us use the same frameworks we used in the _public_ frontend:

- Angular 14
- Bootstrap v5
- NgBootstrap latest

## Additional (long-term) goals of the rewrite will be:

- [ ] i18n on the admin
- [ ] PWA
- [ ] Mobile-ready design for the most commonly used functionality
- [ ] Optimize and simplify UX for most used functionality, especially configuration :1st_place_medal:
- [ ] Add global search box for finding configurations, events, tickets, categories, subscriptions, reservations, atttendees...
- [ ] Introduce an editor + preview for HTML emails, if enabled
- [ ] Styling customization - a more modern look and feel
- [ ] Redo/review the statistics part, it's currently not that useable (we may need some kind of cross event compare & co, global statistics for allowing better forecasting).

## Steps

we decided to span this task over multiple releases, to release more often and collect feedback from users.
The plan would be to:

1. Port 1:1(ish) the current admin
2. UX for settings, check-in, event management
3. Editor for HTML emails
4. Styling customization
5. Misc improvements

This issue is meant to keep track of the progress