New schedule editor

[rixx]

We're getting a new schedule editor (it actually looks like the frontend schedule!), and also a new build process and everything related. Merging this PR will resolve #736 and hopefully #958.

There's a lot yet to be done, but we're getting there.

To do

• Display current schedule
• Schedule and undschedule talks correctly
• Expand timesteps on click
• Handle overlapping talks
• Move editor to a pop-up for more available space
• Create breaks (with a permanent generic unassigned break block)
• Push changes to pretalx
• Scroll while dragging talks
• make sure we have no polling race conditions
• Move breaks
• Run vite and django runserver from the same command
• Document new build requirement for developers and administrators (node 14)
• Bug: broken with empty schedule and/or 1-day event
• Refuse to put sessions on top of each other
• Edit breaks
• Show speaker availabilities while dragging
• Show warnings (aka: use different API)
• Show new warnings on updates
• Support i18n
  • Build translation files including JS strings
  • Add translation files to translate.pretalx.com
  • Set language on page load
  • Moment locale
• Don't pull in all of moment.js
• Add filter/search to unassigned column

Current bugs:

• Drag-select is buggy, finds incorrect slices
• Not all start times are available for alignment/drag-select
• Scheduling unscheduled talks from the sidebar is broken again
• Schedule background colour only applies to initially visible area, disappears on scroll
• Lower half of larger slices can't be hover-selected. Not sure how to fix - try a bit further up, basically?
• breaks forget their length when getting unscheduled
• but-tabs not showing up
• cursor "cannot drp" indicator only works whne not above dragged talk
• Talk drag is offset during side scroll

Nice to haves:

• Schedule release overlay (probably an issue for later)
• Change details in editor: time, date, room, duration (needs to block overlaps, though!)
• Warnings summary in a corner somewhere?
• Find talks (by title, speaker)
• Filter by track/session type
• Make backend reject overlapping sessions
• Filter out rooms, super useful for very wide schedules
• Allow to reorder unassigned talks by track/duration/alnum
• re-add "look at past version" / "reset to past version"

Really great to have:

• multi select talks with shift or rectangle select
• Show distances to other talks, maybe even snap to neighbouring sessions?
• "strict mode" where warnings are errors