feat: added product tour to notify all learners

[sundasnoreen12]

INF-1930

Description

This PR introduces a product tour for the newly added “Notify all learners” checkbox in both the Discussions MFE and the Discussion Sidebar.

The product tour is designed to highlight and explain the new checkbox feature.

Eligibility: The tour will only be shown to users with roles that have permission to view the checkbox.

Dismissal: Once the user acknowledges the tour, it will not appear again.

Content:

Title: "Let your learners know."

Body: "Check this box to notify all learners."

Feature Flag:
The tour is gated behind the same waffle flag "notifications.enable_post_notify_all_learners" that controls the checkbox.

Screenshots/sandbox (optional):

Merge Checklist

• If your update includes visual changes, have they been reviewed by a designer? Send them a link to the Sandbox, if applicable.
• Is there adequate test coverage for your changes?

Post-merge Checklist

• Deploy the changes to prod after verifying on stage or ask @openedx/edx-infinity to do it.
• 🎉 🙌 Celebrate! Thanks for your contribution.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.24%. Comparing base (7ebdf1b) to head (9ae6584).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #783      +/-   ##
==========================================
+ Coverage   93.17%   93.24%   +0.06%     
==========================================
  Files         161      161              
  Lines        3414     3419       +5     
  Branches      925      921       -4     
==========================================
+ Hits         3181     3188       +7     
+ Misses        215      213       -2     
  Partials       18       18              

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