Improve Validation for login/register

[tuur29]

• [x] Add better validation messages in ServerApp.Viewmodels
• [ ] Show all errors instead of only first
• [ ] Show errors beneath correct field
• [x] Check ViewModel errors locally first

[tuur29]

Check out Microsoft.Toolkit.Uwp.UI.Extensions

[tuur29]

Showing all viewmodel errors and showing them below the correct field isn't necessary.