Student login merge code in notification email

[brianhogg]

Description

Uses the same flag used to decide if we ask for their email or a username in the "Lost Password" page.

Fixes #2755

How has this been tested?

Manually

Screenshots

Checklist:

• [x] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [x] My code has been tested.
• [x] My code passes all existing automated tests.
• [x] My code follows the LifterLMS Coding & Documentation Standards.