HTML rendering support for the Website service (#5)

This PR contains the work done to replace the use of static _HTML_ files with type-safe HTML rendered server-side via **Elementary** through **Hummingbird**.

To provide further details about the work done:

* Added the **Elementary** dependencies.
* Added the `IndexPage` and `ErrorPage` pages, ported from the old HTML boilerplate; removed the static files.
* Added the `RootController` controller serving GET / using the `IndexPage` page, wired into the router.
* Reworked the `NotFoundMiddleware` middleare to render `ErrorPage` page directly; non-notFound errors still propagate.
* the `FileMiddleware` middleware no longer searches for any static `index.html` file.
* Simplified the `StaticFile` enumeration, dropped unused constants and now-unnecessary throws.

Reviewed-on: rock-n-code/loud-amsterdam#5
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Sources/App/App.swift

@@ -19,7 +19,7 @@ struct App {
             ]
         )
 
-        let app = try await application(
+        let app = await application(
             reader: reader
         )