Feature: improving events management

[thor]

Background

Editing events is very bare-bones at the moment, and lacks a lot of features that would make it generally more usable for... anything.

• A created event cannot be edited (without having admin access, of some sort)
  • Incorrect details cannot be changed
  • This applies to invoiced events too, which cannot even be edited from the admin panel 😢
• Poor user experience when adding events
• Cannot add more than seven shifts, but usually that's not even necessary
• No details regarding what invoice an event is part of, or which wage report it was part of

In terms of the code, it can now be swapped over to use parts of CBV, although with inlines and whatnot there'll always be some mess.

Rough plan

• [X] Create a minimum viable CBV-version of the events adding
  • [x] Make it fancier
  • [x] ... but also make it simpler
• [x] Create a minimum viable CBV-version of updating events
  • [x] Decide on how to deal with events that have been invoiced 😨
  • [x] Make events reasonably updateable before invoicing
  • [ ] Allow events to be "unready" to be invoiced, or just "not ready" or "not approved"
  • [x] Add another admin model for events without limitations, separate from current
• [x] Separate the events addition template into adding and editing
• [x] Change up the events overview page
  • [ ] Pagination? 📄
  • [x] Linking to the events edit-page 💾
  • [ ] A calendar? 📆
• [x] Add tests for the fun stuff

[thor]

Conclusion: no need to add pagination right now, because the list isn't yet that long. I'll create a separate issue to track pagination of the events view.

Calendar is still on the todo, but waiting for that is detrimental to anything else. There are some Django apps like django-scheduler that can help with this, but they're somewhat more complex than I really care for. All I need is to display it, and that's about where the similarities end.