[Tags] Confusion over child primary tags treated as secondary tags

### Current Behavior

In some parts of the software child primaries are treated as secondary tags (such as checking fo min max requirements when posting). In other parts they are considered primaries (such as checking global start discussion permission). The difference in behavior is very confusing and needs to be organized/figured out.

### Steps to Reproduce

1. in tag settings page set primary to min 1, max 1, set secondary to min 1, max 3
2. create a primary tag and drop it under the primary General
3. create a discussion select both General and the newly created sub tag
4. discussion is created

### Expected Behavior

Needs to be figured out.

### Screenshots

_No response_

### Environment

- Flarum version: Since forever.

### Output of `php flarum info`

_No response_

### Possible Solution

_No response_

### Additional Context

@jordanjay29 and/or @askvortsov1 might have more context about the current behavior.