feat: Modified discussions FE so that ONLY users with verified emails

[sundasnoreen12]

INF-2037

Description

This PR implements the frontend logic to restrict forum posting (adding posts, responses, or comments) to users with verified email accounts, as per the product requirement.

Key Features:
When a user with an unconfirmed email clicks on “Add a post/response/comment,” they will now see a modal dialog that:

Prompts them to verify their email.

Provides a button to send an account confirmation email.

Informs them to reload the page after verification.

The language and appearance of the dialog follow the design mock provided.

Notes:
This feature is guarded by a backend waffle flag "only_verified_users_can_post".

Screenshot

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

Attention: Patch coverage is 94.44444% with 4 lines in your changes missing coverage. Please review.

Project coverage is 93.75%. Comparing base (750720f) to head (cc5037b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/discussions/empty-posts/EmptyTopics.jsx	80.00%	1 Missing ⚠️
...sions/in-context-topics/components/EmptyTopics.jsx	80.00%	1 Missing ⚠️
...iscussions/post-comments/comments/CommentsView.jsx	66.66%	1 Missing ⚠️
...ussions/post-comments/comments/comment/Comment.jsx	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #789      +/-   ##
==========================================
+ Coverage   93.56%   93.75%   +0.18%     
==========================================
  Files         165      166       +1     
  Lines        3561     3618      +57     
  Branches      957      972      +15     
==========================================
+ Hits         3332     3392      +60     
+ Misses        212      210       -2     
+ Partials       17       16       -1     

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