feat: admin search UI

[SychO9]

Part of the roadmap

Changes proposed in this pull request:

• Adds the ability to search through the available settings and permissions.

• Renames app.extensionData to app.registry.

• Adds an app.generalIndex for manual indexing operations.

• To achieve the search, we:

  • Look up settings and permissions within app.registry (formerly app.extensionData)
  • Store core settings in app.registry before displaying them (see BasicsPage changes)
  • When storing settings or permissions is not possible in app.registry (highly custom settings and permission layouts)
  • The dev can use app.generalIndex to store searchable information about said settings/permissions (see AppearencePage, MailPage, AdvancedPage).

• Additionally, this adds an Admin extender to register settings/permissions instead of directly calling app.registry (formerly app.extensionData).

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)