Topic collection custom IA requirements

### As a Joplin admin, I want publish requirements for topic collections that allow me to nest topic collections under other topic collections, so that I can create many levels within the IA.

We have the affordance to assign topic collections to topic collections. But in #3823, we made it so you have to assign a topic collection to a theme, which seems to contradict the nesting topic collections.

**Acceptance criteria**
- [ ] On a topic collection page, the IA requirement for publishing is:
- on publish, you need to have one of these: **theme** OR **topic collection**