feat: frontend content flexible order priorities

[garygreen]

This allows access to page/controller loaded content when using the Frontend content extender - which allows developers to fully customise the document programatically, e.g. able to see Discussion, Tag, etc.

Previous to this PR this was not possible:

   (new Extend\Frontend('forum'))
        ->content(function(Document $document, Request $request) {
                  // Not possible to get the Discussion, or Tag, or anything that the controller loaded here from the `document`
                 $document->title; // always null, despite viewing a Discussion page.
                 dd($document);
        });

Before - $document - notice title is missing

After - notice Discussion information is filled on the document and are able to access the model data.

Use Cases

This has many - one we are looking at is to add a <meta> SEO noindex tag to our Discussion page if we consider the posts there to have "thin content" value. With this PR, it's easily possible to get acccess to the Discussion document and relevant posts. Prior to this PR, it's not easily possible without manually loading the Discussion model, etc.

Reviewers should focus on:
This potentially introduces a breaking behavioural change if people were relying on the old behaviour. I think it's a minor change though.

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]

Hi, thanks for the PR!

This would fix flarum/issue-archive#199

I don't know yet whether this is BC compatible or not, so need to check 🤔

[garygreen]

@SychO9 It does indeed fix flarum/issue-archive#199 - I hadn't seen that issue up until now, glad there is a discussion for this. This is a simple PR that allows devs to have complete final control over the state of the Document.

It might be a BC like you say, as some may rely on the current behaviour of Document missing lots of values, and expecting it to override stuff that was set. That is quite unlikely though, imo. If it is a concern then it can be documented in an upgrade guide, or this could target 2.0 (how far away is that? hopefully not too far because we need this PR!) 😂

[SychO9]

I have added priorities to the PR so that it's a lot more flexible, any developer can now choose to add content before any other content or after using priority numbers.