refactor: Modal rewrite to use native HTML Dialog element + polyfill

[davwheat]

Fixes flarum/issue-archive#124

Changes proposed in this pull request:

Replaces Bootstrap modals with HTML5 Dialog element and polyfill.

This reduces our bundle, and improves accessibility.

Reviewers should focus on:

Testing extension modal implementations work with these changes.

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.
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[davwheat]

Why is this increasing our bundle 🙈

[askvortsov1]

Hello, thank you for your pull request! In order to speed up future development, keep all work in one place, and take advantage of CI tools that can help us avoid breaking changes, we have moved all Flarum code to a single monorepo at flarum/framework. You can read more about this process in our dev diary. Unfortunately, pull requests can't be carried over to the monorepo, so we have to close all open pull requests. If this pull request is still relevant, please feel free to reopen it over on the monorepo. We apologize for the inconvenience, and hope you will consider contributing to Flarum again in the future.