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/Modules/Application/LegacyAppDelegate.h

@@ -31,7 +31,7 @@
 @protocol LegacyAppDelegateDelegate;
 @class CallBar;
 @class CallPresenter;
-@class PreviewManager;
+@class URLPreviewManager;
 
 #pragma mark - Notifications
 /**
@@ -105,9 +105,6 @@ UINavigationControllerDelegate
 // Associated matrix sessions (empty by default).
 @property (nonatomic, readonly) NSArray *mxSessions;
 
-#warning Move this elsewhere.
-@property (nonatomic, readonly) PreviewManager *previewManager;
-
 // Current selected room id. nil if no room is presently visible.
 @property (strong, nonatomic) NSString *visibleRoomId;