Add Functionality to Confirm Account Email on Signup

[blackjackkent]

Expected Behavior

As a user, when I sign up for RPThreadTracker, I would like to receive an account confirmation email to verify that my email was entered correctly, so that I will not encounter issues later if (for instance) I need to retrieve a forgotten password.

Current Behavior

Currently, when a user completes the signup process, no verification is done to ensure they have entered a valid email address. We should institute an email confirmation flow as part of signup.

Possible Solution

The tracker uses (mostly) out-of-the-box ASP.NET Core Identity for authentication at the moment, so we should be able to make use of existing functionality within that library. https://code-maze.com/email-confirmation-aspnet-core-identity/ looks like a good starting guide for this purpose. We should also be able to use the existing service already set up to manage emails through SendGrid.