refactor: extract `save_xblock_to_user_clipboard` Python API

[kdmccormick]

Description

Previously, the logic for copying an XBlock's OLX to the clipboard was directly in a POST view handler. This commit extracts it into a Python API function.

In order to keep models contained within api.py, we move some logic from the UserClipboard model to the UserClipboardData object and change the UserClipboardSerializer to accept the latter instead of the former. As an unavoidable side effect, this means that the source_context_title of the clipboard will not be available in Django admin.

In addition to following architectural guidelines [1], this change will help me develop the Content Libraries Relaunch, as I need a way to locally test library content reference via copy-paste while the UX for it is still being developed.

[1] https://docs.openedx.org/projects/openedx-proposals/en/latest/best-practices/oep-0049-django-app-patterns.html

Testing

When the PR sandbox is ready, before merging, I will manually verify that copy-paste still works as expected.

[open-craft-grove]

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[open-craft-grove]

Sandbox deployment successful 🚀 🎓 LMS 📝 Studio ℹ️ Grove Config, Tutor Config, Tutor Requirements

[open-craft-grove]

Sandbox deployment successful 🚀 🎓 LMS 📝 Studio ℹ️ Grove Config, Tutor Config, Tutor Requirements

[open-craft-grove]

Sandbox deployment successful 🚀 🎓 LMS 📝 Studio ℹ️ Grove Config, Tutor Config, Tutor Requirements

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.