Update DIP-31

[ichub]

Description

We are updating the content of DIP-31 so that it

1. includes the technical specification of our ticketing system
2. clarifies the presence of a two-way sync mechanism between Zupass and Pretix, which keeps whether a particular ticket has been used for check-in synced between the two systems of record.
3. updates all instances of PCDPass to Zupass, as that is what we're calling the application going forward
4. updates technical detail about ticket pcds - we're using eddsa tickets and not rsa tickets

Related DIP issue

https://forum.devcon.org/t/dip-regarding-pcd-based-ticketing-for-devconnect/3201

Checklist

• [x] Correct markdown (e.g. headers with ##, header surrounded by ---)
• [x] syntax check
• [x] filename is DIP-X.md (with X being the correct DIP number)
• [x] header contains (in this order)
  • [x] DIP
  • [x] Title
  • [x] Status
  • [x] Themes
  • [x] Tags
  • [x] Authors
  • [x] Resources Required (optional)
  • [x] Discussion (with link to devcon forum)
  • [x] Created (in the format year-month-day e.g 2020-4-20)
• [x] contains these sections:
  • [x] Abstract
  • [x] Rationale
  • [x] Implementation
  • [x] Operational Requirements & Ownership
• [ ] PR title prefixed with DIP-X

[ligi]

Sorry for the late re-review. Would merge this now - but seems the referenced images are missing.