Update priority of login redirect and limit to My Courses page

[donnapep]

Resolves #7747 and #2823.

Proposed Changes

• In teacher_login_redirect, only redirect to WP admin if the user is logging in from the My Courses page. This solves Admin users are redirected to wp-admin when logging in via My Account page. #2823.
• Updates the priority of the wp_login to 99 so that other code has the opportunity to hook into this filter as well. This still means hooks with a higher priority than 99 won't run, but this solution should catch the majority of cases. This solves Sensei's redirect after login breaks other functionality #7747.

Testing Instructions

1. Log in as a teacher on the My Courses pages.
2. Ensure you're redirected to the dashboard.
3. Login in as an admin on the My Courses pages.
4. Ensure you're redirected to the dashboard.
5. Activate WooCommerce.
6. Go to the My Account page.
7. Log in as an admin.
8. Ensure you stay on the My Account page.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• Code is tested on the minimum supported PHP and WordPress versions

[donnapep]

@m1r0 Tagging you for a quickie review in case I may be missing something. When I removed the filter, the student was still redirected to the admin. But they weren't being redirected back in 2015. So I suppose something changed between now and then in WordPress Core?

UPDATE: Ah, I see it's when they log in via the My Courses page. Need to rework.

[donnapep]

@m1r0 Updated the code to resolve two issues. 🙏🏻

[m1r0]

LGTM! Thanks, Donna!