Add prominent warning on final step of sailor registration

[openwebsolns]

Description

New student registrants often forget the final step of the registration process, where they're asked to identify their pre-existing sailor record, added to enable migration from external source of truth. Despite the messaging at the top of the page:

To finish registration, we need to match you with our records. Click "This is me!" next to the sailor record below that belongs to you. If none match, use the "I'm new!" button at the bottom to proceed.

and despite being the only page available to those accounts when logging-in, many new student registrants complain that they can't find their sailor record despite having successfully registered.

Proposed solution

Add a "warning" paragraph at the top of the page (using XWarning) with messaging like:

You're not done yet! Identify your record below or select "I'm new!" at the bottom of the page!