Ember upgrade! 💖

[compwron]

• Migrate away from ember-cli-pickadate to https://github.com/adopted-ember-addons/ember-pikaday
• Remove bower and migrate dependencies to npm
• Migrate at-js to https://github.com/zurb/tribute#installing

[Tonkpils]

I have a PR that moves us to the lastest 2x LTS #657. The failing CI is from Node versions 16 and above which is expected until we upgrade further.

I'll create separate PRs to upgrade other dependencies slowly and keep a running task list of requirements here.

I also have a draft PR for the upgrade to v3 #660 but that will require more work since we have addons that don't support v3 and are very outdated.