Fix threading in RepositoryHosts log in/out.

Make sure the observables are signalled on the UI thread.

Author: grokys

src/GitHub.App/Models/RepositoryHosts.cs

@@ -180,6 +180,7 @@ public IObservable<AuthenticationResult> LogInFromCache(HostAddress address)
                 var host = await RepositoryHostFactory.Create(address);
                 return host.LogInFromCache()
                     .Catch<AuthenticationResult, Exception>(Observable.Throw<AuthenticationResult>)
+                    .ObserveOn(RxApp.MainThreadScheduler)
                     .Do(result =>
                     {
                         bool successful = result.IsSuccess();
@@ -199,6 +200,7 @@ public IObservable<Unit> LogOut(IRepositoryHost host)
             var address = host.Address;
             var isDotCom = HostAddress.GitHubDotComHostAddress == address;
             return host.LogOut()
+                .ObserveOn(RxApp.MainThreadScheduler)
                 .Do(result =>
                 {
                     // reset the logged out host property to null