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/Tests/Library/Cases/Public/Middlewares/NotFoundMiddlewareTests.swift

@@ -0,0 +1,77 @@
+import Hummingbird
+import HummingbirdTesting
+import NIOCore
+import Testing
+
+@testable import WebsiteCore
+
+@Suite("NotFoundMiddleware middleware")
+struct NotFoundMiddlewareTests {
+    
+    // MARK: Constants
+    
+    private let app: Application = .init(router: {
+        let router = Router()
+        
+        router.addMiddleware {
+            NotFoundMiddleware()
+        }
+        
+        router.get("hello") { _, _ in
+            "Hello!"
+        }
+
+        router.get("boom") { _, _ -> String in
+            throw HTTPError(.badRequest)
+        }
+
+        return router
+    }())
+    
+    // MARK: Functional tests
+
+    @Test
+    func `renders the error page for an unmatched request`() async throws {
+        try await app.test(.router) { client in
+            try await client.execute(
+                uri: "/this-path-does-not-exist",
+                method: .get
+            ) { response in
+                let body = String(buffer: response.body)
+
+                #expect(response.status == .notFound)
+                #expect(response.headers[.contentType] == "text/html; charset=utf-8")
+                #expect(body.contains("Page Not Found"))
+            }
+        }
+    }
+
+    @Test
+    func `passes a matched response through untouched`() async throws {
+        try await app.test(.router) { client in
+            try await client.execute(
+                uri: "/hello",
+                method: .get
+            ) { response in
+                #expect(response.status == .ok)
+                #expect(response.body == ByteBuffer(string: "Hello!"))
+            }
+        }
+    }
+
+    @Test
+    func `rethrows a non-not-found error unchanged`() async throws {
+        try await app.test(.router) { client in
+            try await client.execute(
+                uri: "/boom",
+                method: .get
+            ) { response in
+                let body = String(buffer: response.body)
+                
+                #expect(response.status == .badRequest)
+                #expect(!body.contains("Page Not Found"))
+            }
+        }
+    }
+
+}