Device dehydration v2

- add support for device dehydration v2 i.e. `org.matrix.msc3814`
- run dehydration flows after successfully recovering or creating secrets
- enable said flows based on .well-known `org.matrix.msc3814` config key
- delete previous implementation and helper methods

Riot/Modules/MatrixKit/Controllers/MXKAuthenticationViewController.h

@@ -305,15 +305,5 @@
  */
 - (void)showAuthenticationFallBackView;
 
-#pragma mark - Device rehydration
-
-/**
- Call this method at an appropriate time to attempt rehydrating from an existing dehydrated device
- @param keyData Secret key data
- @param credentials Account credentials
- */
-
-- (void)attemptDeviceRehydrationWithKeyData:(NSData *)keyData credentials:(MXCredentials *)credentials;
-
 @end