Add pretix and programapi mocks to allow isolated development/testing

[NMertsch]

Currently, the bot has three dependencies on external services:

• Discord (obviously)
• Pretix
• programapi

Let's make it easier to develop and test the bot in isolation.

1. Discord: With scripts/configure-guild.py, everyone can create their own Discord server and configure it accordingly.
2. Pretix: Let's add a simple web application which provides a few mock tickets with different properties (e.g. paid/unpaid, business/dinner/childcare/speaker, ...)
3. programapi: Let's add a simple web application which serves a static mock schedule, e.g. a subset of the current EP2025 schedule.

And let's add a step-by-step guide on how to configure everything:

• Create Discord server
• Create Discord application
• Add Discord application to server
• Configure server
• Run Pretix and programapi mocks
• Run bot

[NMertsch]

Reopening, #222 is only a partial resolution.