fix: add viewPrivate visibility scope

[imorland]

**Fixes #3594 **

Changes proposed in this pull request:
Allow discussions to be conditionally visible via viewPrivate scopers, when the user does not have the editPosts permission.

I feel that improving the tests for flarum/approval is also required, unfortunately I cannot run them locally atm, so perhaps someone would be kind enough to add?

Reviewers should focus on:
I've tested this change with 3rd party extensions, such as fof/byobu, which also makes use of viewPrivate, and there does not appear to be any side effects, but perhaps there's something I've not considered by opening this up a little?

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[SychO9]

Looked through the code a bit and from what I can understand, the idea of that block of code in the scoper is to hide discussions where the first post is private (waiting approval or hidden or anything else of the sort).

In which case I think it would be better to drop the editPosts scope completely and instead modify the logic of the first block of code in the scoper (view private logic).

However, whatever solution we try t go with, we need to setup the necessary related tests first.

[SychO9]

I set up the integration tests for the behavior we want to keep and the behavior we want to fix, so one of the tests currently naturally fails (even with the initial fix). This will make it easier to dig into the current logic and how we might want to tweak it.

[askvortsov1]

Was this intended to be marked as ready for review?

[SychO9]

@askvortsov1 nope! not sure how that happened 😅