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/App/App+build.swift

@@ -69,7 +69,7 @@ func application(
 // MARK: - Helpers
 
 // Request context used by application
-private typealias AppRequestContext = BasicRequestContext
+private typealias AppRequestContext = WebsiteRequestContext
 
 /// Builds the cache-control policy applied to the served static files.
 ///
@@ -153,7 +153,8 @@ private func logger(
 /// Registers the request-logging middleware, the security-headers middleware that stamps the given
 /// `securityHeaders` onto every response, the response-compression middleware that compresses
 /// responses larger than `minimumResponseSizeToCompress` when the client advertises support, the
-/// not-found middleware that serves the error page, and the static file middleware that serves the
+/// localization middleware that negotiates the request's language from its `Accept-Language` header,
+/// the not-found middleware that serves the error page, and the static file middleware that serves the
 /// contents of `staticFilesPath` (tagging responses with the given `cacheControl` directives), then
 /// adds the `RootController` routes that render the landing page and the `HealthController` routes
 /// that serve the health check.
@@ -185,6 +186,7 @@ private func router(
         ResponseCompressionMiddleware(
             minimumResponseSizeToCompress: compressionMinResponseSize
         )
+        LocalizationMiddleware()
         NotFoundMiddleware()
         FileMiddleware(
             staticFilesPath,