Fix slow login response and verify email using regex

This week I got my first PR thanks to Flyznex, he helped fix issue #234 Login has a very slow response and along implemented a better way to verify if a string is a valid email. 

The issue came up when I found that sometime when I login it took a long time.  I'd put my email/username and password in and click on the login button and nothing would happen, and after a while like a few seconds then it let me in.

PasswordSignInAsync

As Flyznex pointed in the PR, the SignInManager<TUser> class in Asp.net Core Identity has two PasswordSignInAsync methods, one that takes in a string for userName while the other takes in a user object.  If you call the one that takes in the userName, the method will call UserManager.FindByNameAsync(userName); to look up the user first by its userName, hence you make an extra call to the database.

In my Login method I already looked the user up with its userName by calling my UserService method FindByEmailOrUsernameAsync(loginUser.UserName); so I could just pass the user object to PasswordSignInAsync.  My mistake was that I passed in user.UserName instead thus incurred the extra db call.

Verify if a string is valid email

On my login I allow user to use either either user name or email to login.  When a user is logging in I first check if what's inputted is a valid email or not.  If the string is not a valid email then the user is passing in his user name.

So checking if a string is email sounds easy but in reality it is very complex!  If you don't believe me just take a look at Phil Haack's post I Knew How To Validate An Email Address Until I Read The RFC.  Luckily there is a solution provided to us in the .NET documentation How to: Verify that Strings Are in Valid Email Format.  And when you take a look at that implementation it's like 60 lines of C# code just to do this verification, for good reasons.

Before this PR I was using System.Net.Mail.MailAddress class constructor to verify the validity of an email

// bad code, do not use! 
bool isEmail;              
try  
{  
    new MailAddress(emailOrUsername);  
    isEmail = true;  
}  
catch (FormatException)  
{  
    isEmail = false;  
} 

I originally thought good, less than 10 lines of code to get the same thing done.  But this implementation is actually not reliable.  Try pass in this string a@a as input which is not a valid email, it actually returns true!

Another thing to notice is that verifying email feels like a utility feature, as the name of the class in the .NET doc also suggests RegexUtilities.  For that normally we want to use a static method, but this implementation depends on a local field bool invalid = false; that is shared between the two methods inside the class.  But Flyznex was able to fix that by removing this local variable, as a result our implementation now is really simple to use, you can just call Util.IsValidEmail("email.to.verify@test.com").