Remove requirement to assign team member to create Discussion

[klyburke]

@tima101 In the SaaS demo app, a new user (Team Leader) is unable to create a Discussion before adding additional Team Members. Discussions currently require that someone, besides the creator of the Discussion, is assigned to that Discussion.

Here is what happens when a Team Leader tries to create a Discussion without assigning anyone:

I suggest we remove this requirement. That way, a Team Leader can create a Discussion on their own, without any other Team Members.

[tima101]

@klyburke Pushed https://github.com/async-labs/saas/commit/1696e55402cf8f91fde87803a880cd8f1f7ce8a0 and deployed to https://saas-app.builderbook.org. Clarified in the book as well.