refactor: Simplify client-side email validation

We don't need to implement custom check for lowercase, as it's
been already implemented in joi library. Also we don't need to
convert email to lowercase twice (via onChange and onInput event
handlers), onChange is completely sufficient and as it's just
before form validation, it's the better one.

client/src/Validation/validation.js

@@ -71,13 +71,7 @@ const credentials = joi.object({
 		.string()
 		.trim()
 		.email({ tlds: { allow: false } })
-		.custom((value, helpers) => {
-			const lowercasedValue = value.toLowerCase();
-			if (value !== lowercasedValue) {
-				return helpers.message("Email must be in lowercase");
-			}
-			return lowercasedValue;
-		})
+		.lowercase()
 		.messages({
 			"string.empty": "authRegisterEmailRequired",
 			"string.email": "authRegisterEmailInvalid",