[1.x] [extensibility] refactor(core): improve extensibility of IndexPage

[DavideIadeluca]

Fixes #0000

Changes proposed in this pull request:
Improve extensibility in the frontend to allow third-party extensions to more easily customize this component.

Reviewers should focus on:

• Verify that the backwards compatibility is working as expected. The DOM must be exactly the same.
• A similar PR against 2.x which considers the new PageStructure would probably be desirable. Opinions?

Screenshot
No differences

QA

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)