Fix registration for setups with one organization

[philippotto]

This PR removes the (mainly superfluous) defaultOrganization config from application.conf and instead makes sure that getDefaultOrganization is used where it makes sense.

I think, there is further potential for clean up. I'll create another issue for that.

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• test registration in the following scenarios:
  • isDemoInstance is true
  • isDemoInstance is false (only one organization should be present)

Issues:

• fixes Signing up for default organization is broken #4558

---

• Updated changelog
• Updated migration guide if applicable
• Ready for review

[fm3]

Works for me 🎉 Thanks for the quick yet clean fix :)