Refactor frontend code to allow for extension of assets

- Simpler class naming:
    Frontend\CompilerFactory → Frontend\Assets
    Frontend\HtmlDocumentFactory → Frontend\Frontend
    Frontend\HtmlDocument → Frontend\Document

- Remove AssetInterface and simply collect callbacks in Frontend\Assets
  instead

- Remove ContentInterface because it serves no purpose (never type-
  hinted or type-checked)

- Commit and add asset URLs to the Document via a content callback
  instead of in the Document factory class itself

- Add translations and locale assets to Assets separate to the assets
  factory, as non-forum/admin asset bundles probably won't want them

- Update Frontend Extender to allow the creation of new asset bundles

- Make custom LESS validation listener a standalone class instead of
  extending RecompileFrontendAssets

src/Settings/OverrideSettingsRepository.php

@@ -21,7 +21,7 @@ namespace Flarum\Settings;
  * Within Flarum, this can be used to test out new setting values in a system
  * before they are committed to the database.
  *
- * @see \Flarum\Forum\RecompileFrontendAssets For an example usage.
+ * @see \Flarum\Forum\ValidateCustomLess For an example usage.
  */
 class OverrideSettingsRepository implements SettingsRepositoryInterface
 {