Improve check for existing email addresses

[k3KAW8Pnf7mkmdSMPHz27]

Background
When a user attempts to register a new account, the provided e-mail address is checked against the existing e-mails in the database to detect if there already exists an account with that e-mail address.

The proposed changes

1. Perform the e-mail address comparison in lower-case to make it case-insensitive
2. If there are two or more users with the same e-mail address, return a 400 error (user already exists) instead of a 500 error (internal server error)

[k3KAW8Pnf7mkmdSMPHz27]

@Aadesh-Baral not sure what requirements you have, but if I'd redo this code today (i.e., three years later), I'd either use casefold or normalized Unicode (see https://docs.python.org/3/howto/unicode.html#comparing-strings for background).

On the other hand, since this has been fine for three years, it is unlikely it needs fixing.

Motivation for the original PR:
I was looking at mobile usability. Mobile phones often force an upper-case letter for the first character in an e-mail.
It is, fortunately, less of a problem these days when password managers are omnipresent.

[Aadesh-Baral]

@k3KAW8Pnf7mkmdSMPHz27 Thank you for your comment! I understand your suggestion to use casefold or normalized Unicode for string comparison. While those approaches could potentially fine-tune the code, as you mentioned, it's not a high priority since the function in question isn't used frequently.

However, if you would like to create another pull request to implement the changes you suggested, I would be more than happy to review it. It's always good to consider improvements, especially if they can enhance the code's compatibility or robustness. Feel free to submit the PR.