Allow tests for Extracted & Builtin Discussion XBlock

[salman2013]

Description:

Steps to do in this PR:

• Enable extracted discussion xblock in settings
• Replace pip-installed version of xblocks-contrib with the extracted discussion xblock's PR.
• After testing/fixing all discussion related tests disable extracted discussion xblock in settings.

Acceptance Criteria

• All tests discussion related tests should be green.
• extracted discussion xblock should be disabled in settings.

PR : #37875