Implement RSS feed proxying

SignalServiceKit/src/Loki/API/LokiHttpClient.swift

@@ -1,6 +1,6 @@
 import PromiseKit
 
-/// Base class for `LokiSnodeProxy`, `LokiFileServerProxy` and `LokiRSSFeedProxy`.
+/// Base class for `LokiSnodeProxy` and `LokiFileServerProxy`.
 internal class LokiHTTPClient {
 
     internal lazy var httpSession: AFHTTPSessionManager = {

SignalServiceKit/src/Loki/API/LokiRSSFeedProxy.swift

@@ -0,0 +1,26 @@
+import PromiseKit
+
+internal enum LokiRSSFeedProxy {
+
+    internal enum Error : LocalizedError {
+        case proxyResponseParsingFailed
+
+        internal var errorDescription: String? {
+           switch self {
+           case .proxyResponseParsingFailed: return "Couldn't parse proxy response."
+           }
+        }
+    }
+
+    internal static func fetchContent(for url: String) -> Promise<String> {
+        let server = LokiStorageAPI.server
+        let endpoints = [ "messenger-updates/feed" : "loki/v1/rss/messenger", "loki.network/feed" : "loki/v1/rss/loki" ]
+        let endpoint = endpoints.first { url.lowercased().contains($0.key) }!.value
+        let url = URL(string: server + "/" + endpoint)!
+        let request = TSRequest(url: url)
+        return LokiFileServerProxy(for: server).perform(request).map { response -> String in
+            guard let json = response as? JSON, let data = json["data"] as? String else { throw Error.proxyResponseParsingFailed }
+            return data
+        }
+    }
+}