Localization support for the Website service (#10)

This PR contains the work done to introduce server-side localization support to the **Website** service so the landing and error pages are served in the visitor's negotiated language, backed by a new reusable Localization package.

To provide further details about the work:

* Created the _Localization_ package — a bundle-bound `Localize` method and a `LanguageList` type.
* Language negotiation — `NegotiateLanguage` method picks the best supported language from the request's _Accept-Language_ header (falling back to the default); the `LocalizationMiddleware` middleware resolves it per request and stores it on a new `LocalizedRequestContext` / `WebsiteRequestContext` context.
* Localized responses — `LocalizedHTMLCollectionResponse` pre-renders each page once per language and caches the bytes (with `Content-Language` + `Vary: Accept-Language`), reused by the  `RootController` controller and `NotFoundMiddleware` middleware.
* The `CachedHTMLResponse` response gained custom-header support.
* Localized pages — the `IndexPage` and `ErrorPage` pages now resolve their strings via `Localize` method;
* Added `Localizable.xcstrings` catalogs.
* Wired the `LocalizationMiddleware` middlewaer into the router.

Reviewed-on: rock-n-code/loud-amsterdam#10
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Sources/Library/Public/Middlewares/NotFoundMiddleware.swift

@@ -4,22 +4,24 @@ import Hummingbird
 ///
 /// Placed ahead of `FileMiddleware` in the middleware chain, it catches the `.notFound` error
 /// that bubbles up when no file exists for the requested path and responds with the rendered
-/// ``ErrorPage`` and a `404 Not Found` status.
-public struct NotFoundMiddleware<Context: RequestContext> {
+/// ``ErrorPage`` and a `404 Not Found` status. The page is served in the language stored on the
+/// context by ``LocalizationMiddleware``, falling back to the default language.
+public struct NotFoundMiddleware<Context: LocalizedRequestContext> {
 
     // MARK: Properties
 
-    /// The error page, rendered once at initialization and reused for every not-found response.
-    private let cache: CachedHTMLResponse
+    /// The error page, rendered once per supported language and reused for every not-found response.
+    private let responses: LocalizedHTMLCollectionResponse
 
     // MARK: Initializers
 
     /// Creates a not-found middleware.
     public init() {
-        self.cache = .init(
-            status: .notFound,
-            ErrorPage()
-        )
+        self.responses = .init(
+            status: .notFound
+        ) {
+            ErrorPage(locale: $0)
+        }
     }
 
 }
@@ -56,7 +58,9 @@ extension NotFoundMiddleware: RouterMiddleware {
                 throw error
             }
 
-            return cache.response()
+            return responses.response(
+                for: context.language
+            )
         }
     }