Update dependencies to Flarum 2.0 compatible versions.

Update the extension infrastructure

Importing modules from an extension can now be done though `ext:vendor/extension/module-path` format.

Some Flarum modules are now lazy loaded. Extending them requires a different approach.

Miscellaneous frontend changes

Miscellaneous backend changes

…views

You now need to provide both plain text and HTML views for notification emails.

Flarum 2.0 completely changes the JSON:API implementation

Flarum 2.0 introduces a search driver implementation and moves gambits to the frontend.

Flarum 2.0 uses PHPUnit 11 and encourages use of model factories for easier cross-database testing.

Many variables have been renamed to light/dark specific names and most are now used as CSS variables instead.