Refactoring and tidy up.

Make the preview manager a singleton (passing in the MXSession to functions). Fix tests.

PreviewManager → URLPreviewManager
URLPreviewViewData → URLPreviewData
URLPreviewCache → URLPreviewStore

Riot/Managers/URLPreviews/ClosedURLPreview.swift

@@ -14,9 +14,12 @@
 // limitations under the License.
 //
 
-import Foundation
+import CoreData
 
 extension ClosedURLPreview {
-    // Nothing to extend, however having this file stops Xcode
-    // complaining that it can't find ClosedURLPreview.
+    convenience init(context: NSManagedObjectContext, eventID: String, roomID: String) {
+        self.init(context: context)
+        self.eventID = eventID
+        self.roomID = roomID
+    }
 }